Division and Destiny type of multiplayer.

I wouldn’t consider these two games being a traditional mmo, but how did it allow players to be solo in the game and allow players to join. Would this be done via lobbies/sessions?

It’s just automatically connecting players together in certain areas, just bypassing the matchmaking UI process

Something like the seamless server travel could be used to replicate this I am assuming. I guess my next question is if using a dedicated server is there any way to open a new dedicated server instance or can the dedicated server handle more than one levels/lobbies?