Nothing known about any more then 3 for explore. I would assume the answer is no because, while I enjoy hanging out with more friends, 6 players in constant coordination makes the area too easy. If I could give my suggestion, it would be "linking". Linking would essentially be two seperate fireteams with full voice chat connection that stay together when in the same instances (when you aren't in the same area, you aren't connected). I haven't fleshed out the details, but a "less connected" fireteam of two groups of three would be a nice solution to friends connecting vs too easy gaming. Some Strikes, All Raids, and certain Crucible gamemodes are 6v6.