Get a dedicated server - they're not that expensive - and move your map there (there's instructions on how to do that online). Then your friends can play even when you're offline. My brother-in-law and I did that and it's always a lot of fun to log on, look around, see how, suddenly, everything in our main base looks different and go "oh ... someone went on another building spree ..." :D Like, you leave Monday and there's still wooden palisades and you log on again a week later (cos you're too busy to play more) and suddenly, there's that huge-ass stone wall around the base.