I think the game relies on having good team-mates. That is the nature of Halo, always has been. Having guests in the game places deter in the game, it isn't, then, matchmade.
But enough about that, have good team-mates, have a TEAM, and do call-outs. Call out if you are being shot, call out if you throw a grenade, if you do anything, call out. If you want grenades placing anywhere, call out.