That is the tower, which acts like a social hub - it is the most "MMO" part of the game. Randoms are in the tower, but there is no combat - just mission pickups, turn-ins and vendors.
However Bugnie does not call this an MMO. It certainly takes inspiration from and takes the best aspects of an MMO and incorporates it into the game. However you never will see people in a story mission - that is instanced off from the rest of the world. You may encounter some other people in the patrol/explore mode but that is generally because a public event is coming and those guardians can not hurt you. Strikes (which are like 3 man instances group/co-op content) are matchmade as well as the PVP which is all matchmade.
Plus there is no subscription fee. So like i said - takes the best parts of an MMO