Well, I've never done this myself, but assuming your members are being entered in a DB, you could add a field called newest member, which is either on or off, and only one person can be switched on, which would be done at sign up or activation or whatever.
Alternatively, you could include a date stamp for their signup date and take the highest value there. Hey, you could even date stamp it in Unix time and subtract it from the current unix time and take the smallest result...
Sorry, were you looking for code? I'm just brainstorming...