diff --git a/internal/backend.go b/internal/backend.go
index 5db6f7b..fe020f0 100644
--- a/internal/backend.go
+++ b/internal/backend.go
@@ -47,4 +47,5 @@ type User interface {
 	SetRoomVisibility(Room, createroom.VisibilityType) *models.ApiError
 	Logout(token string)
 	LogoutAll()
+	JoinRoom(Room) *models.ApiError
 }
diff --git a/internal/backends/memory/backend_test.go b/internal/backends/memory/backend_test.go
index 31d08f4..c24877c 100644
--- a/internal/backends/memory/backend_test.go
+++ b/internal/backends/memory/backend_test.go
@@ -136,3 +136,44 @@ func TestGetUserByName(t *testing.T) {
 		assert.Nil(t, user2)
 	})
 }
+
+func TestPublicRooms(t *testing.T) {
+	backend := NewBackend("localhost")
+
+	user1, _, err := backend.Register("user1", "", "")
+	assert.Nil(t, err)
+	assert.NotNil(t, user1)
+
+	// Create first room
+	request := createroom.Request{
+		RoomAliasName: "room1",
+		Name:          "room1",
+		Preset:        createroom.PublicChat}
+
+	room1, err := user1.CreateRoom(request)
+	assert.Nil(t, err)
+	assert.NotNil(t, room1)
+
+	// Create second room
+	request = createroom.Request{
+		RoomAliasName: "room2",
+		Name:          "room2",
+		Preset:        createroom.PublicChat}
+
+	room2, err := user1.CreateRoom(request)
+	assert.Nil(t, err)
+	assert.NotNil(t, room2)
+
+	// Make room2 has 2 users
+	user2, _, err := backend.Register("user2", "", "")
+	assert.Nil(t, err)
+	assert.NotNil(t, user2)
+
+	err = user2.JoinRoom(room2)
+	assert.Nil(t, err)
+
+	rooms := backend.PublicRooms()
+	assert.Len(t, rooms, 2)
+	assert.Equal(t, rooms[0], room2)
+	assert.Equal(t, rooms[1], room1)
+}
diff --git a/internal/backends/memory/user.go b/internal/backends/memory/user.go
index 76b95f4..221e0ad 100644
--- a/internal/backends/memory/user.go
+++ b/internal/backends/memory/user.go
@@ -222,3 +222,20 @@ func (user *User) Logout(token string) {
 func (user *User) LogoutAll() {
 	user.Tokens = make(map[string]Token)
 }
+
+func (user *User) JoinRoom(room internal.Room) *models.ApiError {
+	memRoom := room.(*Room)
+
+	memRoom.mutex.Lock()
+	defer memRoom.mutex.Unlock()
+
+	for _, roomUser := range memRoom.joined {
+		if roomUser.ID() == user.ID() {
+			return internal.NewError(models.M_BAD_STATE, "user already in room") // TODO: check code
+		}
+	}
+
+	memRoom.joined = append(memRoom.joined, user)
+
+	return nil
+}