What they said. It expresses your excitement at the prospect of working for them, reinforces that you believe you are a good fit, and ideally recalls or follows up on something from the interview.

And yes, I know this is a less formal situation. The general advice still applies...if you want to nudge them about an answer, thank them for considering you and talk about how excited you are and close by mentioning that you look forward to hearing from them soon. If you had reason to expect an answer and haven't gotten one yet, and you haven't sent them anything recently, a note politely asking if you missed their response can show your interest...I just sent one of those today.