If you're like me and you never played the game until it was on the Steam sale (as it's a @$@* sensational game) you're probably having issues logging into the Rockstar Social Club (assuming you've joined before).
All you need to do is access the Rockstar social club here: http://socialclub.rockstargames.com/
and update your details. it'll let you log into the game from that point on.
Took me ages to find that :( --- Bide your time, and hold out hope...