diff --git a/framework/core/src/Event/PrepareUserGroups.php b/framework/core/src/Event/PrepareUserGroups.php
index 32e13e0db..10757544c 100644
--- a/framework/core/src/Event/PrepareUserGroups.php
+++ b/framework/core/src/Event/PrepareUserGroups.php
@@ -12,6 +12,7 @@ namespace Flarum\Event;
 use Flarum\User\User;
 
 /**
+ * @deprecated beta 13, remove in beta 14. Use the User extender instead.
  * The `PrepareUserGroups` event.
  */
 class PrepareUserGroups
diff --git a/framework/core/src/Extend/User.php b/framework/core/src/Extend/User.php
index 63dd92da5..130f19249 100644
--- a/framework/core/src/Extend/User.php
+++ b/framework/core/src/Extend/User.php
@@ -15,9 +15,10 @@ use Illuminate\Contracts\Container\Container;
 class User implements ExtenderInterface
 {
     private $displayNameDrivers = [];
+    private $groupProcessors = [];
 
     /**
-     * Add a mail driver.
+     * Add a display name driver.
      *
      * @param string $identifier Identifier for display name driver. E.g. 'username' for UserNameDriver
      * @param string $driver ::class attribute of driver class, which must implement Flarum\User\DisplayName\DriverInterface
@@ -29,10 +30,35 @@ class User implements ExtenderInterface
         return $this;
     }
 
+    /**
+     * Dynamically process a user's list of groups when calculating permissions.
+     * This can be used to give a user permissions for groups they aren't actually in, based on context.
+     * It will not change the group badges displayed for the user.
+     *
+     * @param callable $callable
+     *
+     * The callable can be a closure or invokable class, and should accept:
+     * - \Flarum\User\User $user: the user in question.
+     * - array $groupIds: an array of ids for the groups the user belongs to.
+     *
+     * The callable should return:
+     * - array $groupIds: an array of ids for the groups the user belongs to.
+     */
+    public function permissionGroups(callable $callable)
+    {
+        $this->groupProcessors[] = $callable;
+
+        return $this;
+    }
+
     public function extend(Container $container, Extension $extension = null)
     {
         $container->extend('flarum.user.display_name.supported_drivers', function ($existingDrivers) {
             return array_merge($existingDrivers, $this->displayNameDrivers);
         });
+
+        $container->extend('flarum.user.group_processors', function ($existingRelations) {
+            return array_merge($existingRelations, $this->groupProcessors);
+        });
     }
 }
diff --git a/framework/core/src/User/User.php b/framework/core/src/User/User.php
index 48b02b64a..d3832abb5 100644
--- a/framework/core/src/User/User.php
+++ b/framework/core/src/User/User.php
@@ -83,6 +83,12 @@ class User extends AbstractModel
      */
     protected $session;
 
+    /**
+     * An array of callables, through each of which the user's list of groups is passed
+     * before being returned.
+     */
+    protected static $groupProcessors = [];
+
     /**
      * An array of registered user preferences. Each preference is defined with
      * a key, and its value is an array containing the following keys:.
@@ -660,8 +666,13 @@ class User extends AbstractModel
             $groupIds = array_merge($groupIds, [Group::MEMBER_ID], $this->groups->pluck('id')->all());
         }
 
+        // Deprecated, remove in beta 14.
         event(new PrepareUserGroups($this, $groupIds));
 
+        foreach (static::$groupProcessors as $processor) {
+            $groupIds = $processor($this, $groupIds);
+        }
+
         return Permission::whereIn('group_id', $groupIds);
     }
 
@@ -751,6 +762,17 @@ class User extends AbstractModel
         static::$preferences[$key] = compact('transformer', 'default');
     }
 
+    /**
+     * Register a callback that processes a user's list of groups.
+     *
+     * @param callable $callback
+     * @return array $groupIds
+     */
+    public static function addGroupProcessor($callback)
+    {
+        static::$groupProcessors[] = $callback;
+    }
+
     /**
      * Get the key for a preference which flags whether or not the user will
      * receive a notification for $type via $method.
diff --git a/framework/core/src/User/UserServiceProvider.php b/framework/core/src/User/UserServiceProvider.php
index c83fe6ad1..5cb6d8bde 100644
--- a/framework/core/src/User/UserServiceProvider.php
+++ b/framework/core/src/User/UserServiceProvider.php
@@ -31,6 +31,10 @@ class UserServiceProvider extends AbstractServiceProvider
     {
         $this->registerAvatarsFilesystem();
         $this->registerDisplayNameDrivers();
+
+        $this->app->singleton('flarum.user.group_processors', function () {
+            return [];
+        });
     }
 
     protected function registerDisplayNameDrivers()
@@ -72,6 +76,14 @@ class UserServiceProvider extends AbstractServiceProvider
      */
     public function boot()
     {
+        foreach ($this->app->make('flarum.user.group_processors') as $callback) {
+            if (is_string($callback)) {
+                $callback = $this->app->make($callback);
+            }
+
+            User::addGroupProcessor($callback);
+        }
+
         User::setHasher($this->app->make('hash'));
         User::setGate($this->app->make(Gate::class));
         User::setDisplayNameDriver($this->app->make('flarum.user.display_name.driver'));
diff --git a/framework/core/tests/integration/extenders/UserTest.php b/framework/core/tests/integration/extenders/UserTest.php
index 78449b373..1a897c719 100644
--- a/framework/core/tests/integration/extenders/UserTest.php
+++ b/framework/core/tests/integration/extenders/UserTest.php
@@ -24,7 +24,9 @@ class UserTest extends TestCase
         $this->prepareDatabase([
             'users' => [
                 $this->adminUser(),
-            ], 'settings' => [
+                $this->normalUser(),
+            ],
+            'settings' => [
                 ['key' => 'display_name_driver', 'value' => 'custom'],
             ],
         ]);
@@ -58,6 +60,34 @@ class UserTest extends TestCase
 
         $this->assertEquals('admin@machine.local$$$suffix', $user->displayName);
     }
+
+    /**
+     * @test
+     */
+    public function user_has_permissions_for_expected_groups_if_no_processors_added()
+    {
+        $this->prepDb();
+        $user = User::find(2);
+
+        $this->assertContains('viewUserList', $user->getPermissions());
+    }
+
+    /**
+     * @test
+     */
+    public function processor_can_restrict_user_groups()
+    {
+        $this->extend((new Extend\User)->permissionGroups(function (User $user, array $groupIds) {
+            return array_filter($groupIds, function ($id) {
+                return $id != 3;
+            });
+        }));
+
+        $this->prepDb();
+        $user = User::find(2);
+
+        $this->assertNotContains('viewUserList', $user->getPermissions());
+    }
 }
 
 class CustomDisplayNameDriver implements DriverInterface