As Badelaire said, it's the effort of letting a faction now you're interested in them OOCly, to back up IG ground-work you've done. It sounds stupid, but the amount of intel each of the RVT players is privy to, is a lot, which is why it is cliquish, one wrong decision, even though we can ICly correct it, and that information isn't really tradeable, or confidential, if you know what I mean.. and we're one of the easiest factions to get involved with.
There are a few PCs that had interest in joining, and let me know OOCly that they'd be interested in it, after my char had gotten to know theirs semi-decently IG. When the time came, and they sent a message/applied, vouches based off IG behaviour were made, but also, some smaller ones of an OOC nature (such as good loser, immersive, etc - really good qualities to have).