Custom select field with current members listed

I'm looking to build a custom select field that is pre-populated with the current list of forum members. Anyone have any tips to get me started?
Darn. You don't know of any add-ons that are a referral system of any sort, do you? I'm wanting to reward folks for referring others to our forums. I'm not too picky on how it's done but it's something I'm definitely interested in.
I don't mind paying at all and I'd be happy to go ask the author but do you know if it's compatible with the current version of XF? Yeah I'm lazy like that lol.