People in your fire team have their names highlighted in green.
Randoms have their names highlighted in blue.
open world Patrol can have multiple fireteams on the map doing public events.
You can have a Fireteam of 6 though for Raids and for PvP.
How it works
Although Destiny is capable of being played solely, the game has been dubbed a "shared world shooter." Bungie's new matchmaking system will seemlessly connect players to other random players throughout the course of the game. Players then have the option to remain with their newly appointed friends or diverge. It also has up to 6 players in one squad at a time
Strikes: They are very similar to Firefight from Halo 3: ODST and Halo: Reach. They feature a series of waves and objectives which culminates in a boss battle at the end. It is entirely cooperative. At the end, you are given a Strike Rank between Bronze, Silver, and Gold.
Raids: Much more difficult than regular missions. You'll have to be strong and well equipt to face them. Little else is known.