diff --git a/src/db/domain.rs b/src/db/domain.rs
new file mode 100644
index 0000000..5ea6d43
--- /dev/null
+++ b/src/db/domain.rs
@@ -0,0 +1,35 @@
+use crate::db::error::UserCreationError;
+use crate::db::{Group, User, UserRef};
+
+#[derive(Clone, Debug, Default)]
+pub struct Domain {
+    pub name: String,
+    pub users: Vec<User>,
+    #[expect(unused)]
+    pub groups: Vec<Group>,
+}
+
+impl Domain {
+    pub fn get_user(&self, req_user: &UserRef) -> Option<User> {
+        assert!(
+            req_user.domain.as_deref().unwrap_or("") == self.name,
+            "Should only call Domain::get_user on the matching domain. Asked for {:?} on domain {}",
+            req_user,
+            self.name
+        );
+
+        self.users
+            .iter()
+            .find(|u| u.username == req_user.username)
+            .cloned()
+    }
+
+    pub fn create_user(&mut self, user: User, user_ref: UserRef) -> Result<(), UserCreationError> {
+        if self.get_user(&user_ref).is_some() {
+            return Err(UserCreationError::UserAlreadyExists(user_ref));
+        }
+
+        self.users.push(user);
+        Ok(())
+    }
+}
diff --git a/src/db/error.rs b/src/db/error.rs
index ccdcd47..9fdce05 100644
--- a/src/db/error.rs
+++ b/src/db/error.rs
@@ -6,6 +6,7 @@ pub type BoxedError = Box<dyn std::error::Error + Send + Sync + 'static>;
 
 #[derive(Debug)]
 pub enum UserCreationError {
+    DomainNotFound(String),
     UserAlreadyExists(UserRef),
     Permissions,
 }
@@ -13,6 +14,7 @@ pub enum UserCreationError {
 impl fmt::Display for UserCreationError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
+            Self::DomainNotFound(domain) => write!(f, "No domain {domain} to create user in"),
             Self::UserAlreadyExists(user) => write!(f, "User already exists: {user}"),
             Self::Permissions => write!(f, "You do not have permissions to create this user"),
         }
@@ -20,3 +22,25 @@ impl fmt::Display for UserCreationError {
 }
 
 impl std::error::Error for UserCreationError {}
+
+#[derive(Debug)]
+pub enum DomainCreationError {
+    DomainAlreadyExists(String),
+    InvalidDomain(String),
+}
+
+impl fmt::Display for DomainCreationError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::DomainAlreadyExists(domain) => {
+                write!(f, "Cannot create domain {domain} because it already exists")
+            }
+            Self::InvalidDomain(domain) => write!(
+                f,
+                "Cannot create domain `{domain}` because it's not considered a valid domain"
+            ),
+        }
+    }
+}
+
+impl std::error::Error for DomainCreationError {}
diff --git a/src/db/interface.rs b/src/db/interface.rs
index 668e03b..ed6d86c 100644
--- a/src/db/interface.rs
+++ b/src/db/interface.rs
@@ -1,7 +1,18 @@
-use crate::db::error::{BoxedError, UserCreationError};
-use crate::db::{Database, User, UserRef};
+use crate::db::error::{BoxedError, DomainCreationError, UserCreationError};
+use crate::db::{Database, Domain, User, UserRef};
 
 impl<D: DatabaseInterface> DatabaseInterface for Database<D> {
+    async fn create_domain(
+        &mut self,
+        domain: &str,
+    ) -> Result<Result<(), DomainCreationError>, BoxedError> {
+        self.inner.write().await.create_domain(domain).await
+    }
+
+    async fn get_domain(&self, domain: &str) -> Result<Option<Domain>, BoxedError> {
+        self.inner.read().await.get_domain(domain).await
+    }
+
     async fn get_user(&self, user: &UserRef) -> Result<Option<User>, BoxedError> {
         self.inner.read().await.get_user(user).await
     }
@@ -31,6 +42,12 @@ impl<D: DatabaseInterface> DatabaseInterface for Database<D> {
 }
 
 pub trait DatabaseInterface: Clone + Send + Sync + 'static {
+    fn create_domain(
+        &mut self,
+        domain: &str,
+    ) -> impl Future<Output = Result<Result<(), DomainCreationError>, BoxedError>>;
+    fn get_domain(&self, domain: &str) -> impl Future<Output = Result<Option<Domain>, BoxedError>>;
+
     fn get_user(
         &self,
         user: &UserRef,
@@ -45,6 +62,10 @@ pub trait DatabaseInterface: Clone + Send + Sync + 'static {
         new_user: User,
         current_user: &User,
     ) -> Result<Result<(), UserCreationError>, BoxedError>;
+
+    /// Use empty string domain to request global users (TODO: this is not very DX)
+    ///
+    /// When domain does not exist, the returned list is empty.
     fn list_users(
         &self,
         domain: Option<String>,
diff --git a/src/db/memory.rs b/src/db/memory.rs
index 62b98a3..b9b8545 100644
--- a/src/db/memory.rs
+++ b/src/db/memory.rs
@@ -1,46 +1,90 @@
 use std::future::{Future, ready};
 
-use crate::db::error::{BoxedError, UserCreationError};
-use crate::db::{Database, DatabaseInterface, Group, Operation, User, UserRef};
+use crate::db::error::{BoxedError, DomainCreationError, UserCreationError};
+use crate::db::{Database, DatabaseInterface, Domain, Operation, User, UserRef};
 
 #[derive(Clone, Debug, Default)]
 pub struct MemoryDatabase {
-    pub users: Vec<User>,
-    #[expect(unused)]
-    pub groups: Vec<Group>,
+    pub domains: Vec<Domain>,
+    /// Where global users/groups are registered
+    pub global_domain: Domain,
 }
 
 impl MemoryDatabase {
     pub fn new() -> Database<Self> {
         Database::new(Self::default())
     }
+
+    pub fn get_domain_mut(&mut self, domain: &str) -> Option<&mut Domain> {
+        self.domains.iter_mut().find(|d| d.name == domain)
+    }
 }
 
 impl DatabaseInterface for MemoryDatabase {
-    fn get_user(
-        &self,
-        req_user: &UserRef,
-    ) -> impl Future<Output = Result<Option<User>, BoxedError>> {
-        for user in &self.users {
-            if user.username == req_user.username && user.domain == req_user.domain {
-                return ready(Ok(Some(user.clone())));
-            }
+    fn create_domain(
+        &mut self,
+        domain: &str,
+    ) -> impl Future<Output = Result<Result<(), DomainCreationError>, BoxedError>> {
+        if domain.is_empty() {
+            return ready(Ok(Err(DomainCreationError::InvalidDomain(
+                domain.to_string(),
+            ))));
         }
 
-        ready(Ok(None))
+        if self.domains.iter().find(|d| d.name == domain).is_some() {
+            return ready(Ok(Err(DomainCreationError::DomainAlreadyExists(
+                domain.to_string(),
+            ))));
+        }
+
+        self.domains.push(Domain {
+            name: domain.to_string(),
+            users: vec![],
+            groups: vec![],
+        });
+
+        ready(Ok(Ok(())))
     }
 
-    async fn create_user(
+    fn get_domain(&self, domain: &str) -> impl Future<Output = Result<Option<Domain>, BoxedError>> {
+        let Some(domain) = self.domains.iter().find(|d| d.name == domain) else {
+            return ready(Ok(None));
+        };
+
+        ready(Ok(Some(domain.clone())))
+    }
+
+    async fn get_user(&self, req_user: &UserRef) -> Result<Option<User>, BoxedError> {
+        let Some(req_domain) = &req_user.domain else {
+            // If no domain is provided for the user query, look up the global users
+            return Ok(self.global_domain.get_user(req_user));
+        };
+
+        let Some(domain) = self.get_domain(req_domain).await? else {
+            return Ok(None);
+        };
+
+        Ok(domain.get_user(req_user))
+    }
+
+    fn create_user(
         &mut self,
         user: User,
-    ) -> Result<Result<(), UserCreationError>, BoxedError> {
+    ) -> impl Future<Output = Result<Result<(), UserCreationError>, BoxedError>> {
         let user_ref = user.user_ref();
-        if self.get_user(&user_ref).await?.is_some() {
-            return Ok(Err(UserCreationError::UserAlreadyExists(user_ref)));
-        }
 
-        self.users.push(user);
-        Ok(Ok(()))
+        let Some(req_domain) = &user.domain else {
+            // No domain requested, this is a global user creation
+            return ready(Ok(self.global_domain.create_user(user, user_ref)));
+        };
+
+        let Some(domain) = self.get_domain_mut(req_domain) else {
+            return ready(Ok(Err(UserCreationError::DomainNotFound(
+                req_domain.clone(),
+            ))));
+        };
+
+        ready(Ok(domain.create_user(user, user_ref)))
     }
 
     async fn try_create_user(
@@ -65,19 +109,23 @@ impl DatabaseInterface for MemoryDatabase {
         self.create_user(new_user).await
     }
 
-    fn list_users(
-        &self,
-        domain: Option<String>,
-    ) -> impl Future<Output = Result<Vec<User>, BoxedError>> {
-        let users = if let Some(domain) = domain {
-            self.users
-                .iter()
-                .filter(|user| user.domain.as_ref() == Some(&domain))
-                .cloned()
-                .collect()
+    async fn list_users(&self, domain: Option<String>) -> Result<Vec<User>, BoxedError> {
+        if let Some(domain) = domain {
+            if domain.is_empty() {
+                Ok(self.global_domain.users.clone())
+            } else {
+                let Some(domain) = self.get_domain(&domain).await? else {
+                    return Ok(vec![]);
+                };
+                Ok(domain.users.clone())
+            }
         } else {
-            self.users.clone()
-        };
-        ready(Ok(users))
+            // Aggregate all users
+            let mut users = self.global_domain.users.clone();
+            for domain in &self.domains {
+                users.extend(domain.users.clone());
+            }
+            Ok(users)
+        }
     }
 }
diff --git a/src/db/mod.rs b/src/db/mod.rs
index d77daca..c2544e0 100644
--- a/src/db/mod.rs
+++ b/src/db/mod.rs
@@ -1,5 +1,7 @@
 mod common;
 pub use common::Database;
+mod domain;
+pub use domain::Domain;
 pub mod error;
 mod group;
 pub use group::Group;
diff --git a/src/main.rs b/src/main.rs
index ace3805..cb6ee76 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -29,22 +29,24 @@ async fn create_dummy_users<D: DatabaseInterface>(db: &mut Database<D>) {
     .await
     .unwrap()
     .unwrap();
-    for domain in &["a", "b", "c"] {
+    for letter in &["a", "b", "c"] {
+        let domain = format!("{letter}.localhost");
+        db.create_domain(&domain).await.unwrap().unwrap();
         db.create_user(User {
-            username: domain.to_string(),
-            domain: Some(format!("{domain}.localhost")),
+            username: letter.to_string(),
+            domain: Some(domain.clone()),
             password: "adminadmin".to_string(),
-            mail: format!("{domain}@{domain}.localhost"),
-            role: Role::DomainAdmin(format!("{domain}.localhost")),
+            mail: format!("{letter}@{domain}"),
+            role: Role::DomainAdmin(domain.clone()),
         })
         .await
         .unwrap()
         .unwrap();
         db.create_user(User {
-            username: format!("user{domain}"),
-            domain: Some(format!("{domain}.localhost")),
+            username: format!("user{letter}"),
+            domain: Some(domain.clone()),
             password: "adminadmin".to_string(),
-            mail: format!("user{domain}@{domain}.localhost"),
+            mail: format!("user{letter}@{domain}"),
             role: Role::User,
         })
         .await