I've created my Runic Games login, and linked it with my Steam account, so if anyone wants to smash & grab wholesale sometime when I'm logged into Steam then give me a scream.
I've got it all setup too just need to remember my login information.
I'm not sure if there's any kind of Level-scaling between everyone's characters in multi-player. I expect that not to be the case, but then again new zones can be re-generated whenever you want so it's not like you can't redo the stuff whenever you want.
My highest level character is an Embermage @ 36 and a spread of lowbies.