I think it's going to work like some of the recent semi-MMOs announced like Destiny, or The Division. Some players will be allowed into your world but not all of them. You'll more likely than not be able to invite your friends to come along with you.
Slightly disappointing that it won't be live day one, but it's a small price to pay. I didn't think it was possible for me to want this game any harder.
XBL Gamertag: JWRebholz
PSN ID: jwrebholz
...No, I'm not very creative when it comes to this sort of thing.