Finding a fire team for things isn't always the easiest thing. So why not add a second one? On this second bounty board you could post openings for things like the nightfall strike or raids. Also let players set the level you have to be to pick them up. That way people can have a team of people they actually need. You could even set a reward (I.E. glimmer, crucible or vanguard marks, strange coins, motes of light or even one of the players weapons/armor)