Nope, what if you want to make something and keep it secret until it's finished? I'd prefer some kind of radar system. But *crag* that for now, custom items are more important

poops has a point. I say just have a popup say "X is in Y" or something when you click their name in the online members menu. If the level isn't named then just say "in an unnamed level belonging to X" or if they're in a house say "In X's (area) house" where area is either overworld, underground, etc. I don't know what to do if it's a house in a house in a house but I guess you could just say "a room somewhere belonging to X"

Could always have a checkbox that defines whether or not other players can warp to you.

radar still sounds like a better idea.

problem with optional teleportation is nobody will use it then

Actually, a good compromise is a "warp to area player is in" option, which just brings you to the entrance of that area. Unfinished levels are blocked off in their entrance anyway, so there's no risk of letting someone in somewhere they shouldn't be yet.