World Creation (like Minecraft)

Hi. I was wondering how you would go about some form of "world creation" and world joining system.

I've tried using the sessions & gameini's method, but cannot figure out how to save placed blocks, plus, the worlds are really buggy with sessions & gameini's, for example, sometimes when you join one it actually doesn't join, etc.

Anyway, hope I can get a reply soon, thanks :-).