diff --git a/src/auth/AuthGuard.ts b/src/auth/AuthGuard.ts
index 15e5a46..a808a87 100644
--- a/src/auth/AuthGuard.ts
+++ b/src/auth/AuthGuard.ts
@@ -12,7 +12,7 @@ export default abstract class AuthGuard<P extends AuthProof> {
         return await User.getById<User>(session.auth_id);
     }
 
-    public async authenticateOrRegister(session: Express.Session, proof: P): Promise<void> {
+    public async authenticateOrRegister(session: Express.Session, proof: P, registerCallback?: (connection: Connection, userID: number) => Promise<(() => Promise<void>)[]>): Promise<void> {
         if (!await proof.isAuthorized()) {
             throw new AuthError('Invalid argument: cannot authenticate with an unauthorized proof.');
         }
@@ -34,6 +34,9 @@ export default abstract class AuthGuard<P extends AuthProof> {
                     main: true,
                 });
                 await userEmail.save(connection, c => callbacks.push(c));
+                if (registerCallback) {
+                    (await registerCallback(connection, user.id!)).forEach(c => callbacks.push(c));
+                }
             });
 
             for (const callback of callbacks) {
@@ -43,6 +46,8 @@ export default abstract class AuthGuard<P extends AuthProof> {
             if (!user) {
                 throw new Error('Unable to register user.');
             }
+        } else if (registerCallback) {
+            throw new AuthError('User already exists.');
         }
 
         session.auth_id = user.id;