You could do this by creating a sort of hub world to join up into first, a sort of buffer after creating your character. Use the same idea as starting off classles then graduating to a villian or hero later, or like a secondary job. Then control and track the players using variables when they enter or leave the official map. You could also determine their classes when they join and have lobbies leading into rooms and so on. This may not be the style youre going for, but without source thats the only wat I can imagine it being done.