Ehm, i haven’t used it yet, but Raknet should have its own Session System.
If you look into the Manuel of Raknet: Jenkins Software
You will find basics and how they work. For example a plugin to use Steam with it: Lobby2Client_Steam
Or a lobby system without Steam or any other subsystem: LobbyServer and LobbyClient manual
And many other things. But from my side, it is up to you to implement that. I have never done that. :X