1-How about idea of owner can see where is the all persons? when on the menu when you get your mouse to their name the minimap opens and shows him in a red circle.
2-And a Owner God System. A special god mode for "just owners" for the owner of the world. so you can see who is it.
3-A Friend System: How about a friend system? so when other player accepted your friend message he can see the where he is and join to that world (not for beta only worlds)
Add to 3. : A friend world for just YOUR friends
oh hey! cool display pi- *seizure*