I'm positive its something simple but I've been trying to kick one party member out to prevent another from leaving for over an hour now, and I just cant see how to do it!

I knew I'd find it after giving in and posting that! Sorry.

If that was about Annah and Grace, you can actually convince Annah to come back after she leaves, if you have at least 13 Charisma; she will never threaten to leave again after that.. So you can keep both of them around if you know how to handle Annah - she doesn't really like the idea of going back to her corpse scavenging "business"...