I'm making a multiplayer game, where players are separated in to rooms that would ideally have about 20 players.

So I need a few pointers on an algorithm to distribute the players in to these rooms.

A few more constraints:

• When a players gets in to a room, he should stay there until he decides to exit (the room itself changes levels)
• There may be more room servers, every server should create more rooms until near full capacity
• There's a central server that manages all the room servers, and directs the players towards their room