My reason behind thinking it MIGHT be possible is I watched one of the way old pre release gameplay trailers where the dude gets thunderlord after like 2 min into the game...from a yellow engram...dropped by the first archon dude in cosmodrome...it decrypts as soon as he picks it up lol. But the point is after that a public event happens(walker tank) and while their 3 man team is fighting it he comments: "another fireteam droppin in...ooooh they're lvl 20's this is gonna be sick!" I immediately thought how great it would be to allow multiple fireteams into horde mode. TLDR: got idea from watching old pre release gameplay trailer you should go back and watch them and see how awesome some of that stuff would've been if it hadn't been cut.