This is an acronym describing one's involvement at a party when there are girls around. It is normally used by one friend who is not in attendance at said party talking to another friend who is (via text). M.F.O.I. means Mingling, Fingering, or Otherwise Interacting (in reference to the people at the party (mingling), the girls at the party (fingering; could be used literally, but it does not need to be)), or otherwise interacting (etc...).
Guy 1 (not at party): Hey dude, hows the party? There any girls there?
Guy 2 (at party): It's alright, and they ain't bad.
Guy 1 (not satisfied with answer): You M.F.O.I.?
Guy 2: Nah, I'm just hanging with the guys, playin' some beer pong.
Guy 1: C'mon man, play the field.