From the previews I read or podcasts I've listened too, you don't actually have to group up to participate in guests together. That said, I haven't actually played so I have no idea. Still I am excited for it. That and Borderlands 2...and the new Civ5 expansion.
As for SWTOR, I haven't made it to 50 yet, though my smuggler is close. Hard to make alot of progress playing only a few hours at a time. As you pointed out, rolling new char is out for me, I'll wait to see what happens with the server merges...if I'm still playing by the time they come.