Colonists?
 

Just curious what we can use them for.
Also, are we supposed to drop them off at some planet at some point?

You can use them in missions with "Any Officer" slots, and yes there is also a mission to drop them off.

You should have gained 5 in a mission, and you will need 5 for the mission to drop them off, then they will be removed from your list.

I see the mission to drop off the colonists quite frequently... too bad I have only seen a mission to pick up colonists once.