Pick a game, pick a room and hope.

Ask in a lobby if they have seen the person (or even ask on the forum!)

Much easier if your a subsriber to change rooms and find someone ;)

Another bit of advice, when going into rooms, clikc show all players, and click on name, to list them alphabetically, makes finding someone much easier :)

If the person no longer goes FOD i don't believe there's a way to find when they last logged in (a buddy list which tell you where people on it are would be a good idea for FOD )

Hope that helps a little.