Bungie has been cagey about certain aspects of Destiny, particularly in how some aspects of the multiplayer work. In a Q&A, Bungie shared a few tidbits of detail on the competitive multiplayer, grouping up, and how the game will evolve after finishing the campaign.
On the PlayStation.Blog, Bungie's community manager mentioned that any gear you earn in the competitive multiplayer can be taken to the other modes--and vice-versa. So if you've spent hours with your Fireteam in the campaign and built up a strong Guardian, that will pay off when it comes time to prove it by throwing down. That mode is still in internal testing at the studio, though, and they're not ready to show it off.
If you want to make it through the campaign solo, you can--though, naturally, Bungie urges you to team up with other Guardians since the game is balanced that way. Joining up in large events is as easy as being in the right place and firing your weapon, so Bungie says it has stressed making participating in a group as painless as possible.
Once you've finished the campaign, Bungie says the game world will continue to evolve with new activities, new twists on existing game modes, and new raids. It wasn't quite clear on how much of that will be shipped with the game for speedy players, and how much will be added post-launch, but this is taking some heavy influence from MMOs regardless.