I've started coding the lobby, but before that I started cleaning up a bit the UI for the login and do some checks server side (like checking if the username is already taken).
So the lobby will basically be a list of what i call game sessions. Each game session will group together 3 players to play in a level. The user will click on the preferred character type he wishes to play with. When the 3 slots of a game session have been taken, the players will be launched into the level.
So far I've only worked a bit on the UI and put on paper the logic and server protocol. Tomorrow it will be implementation day :)