As already mentioned Sussexmtb are your nearest for regular group rides, also consider Brightonmtb.org as we often ride South Downs in your area too - weekly night rides from Sussex Uni at Falmer and twice monthly rides all over the area, inc Horsham and Tilgate which I lead, and Whiteways to name just a few. Both groups are friendly and often intertwine for rides.
I'll be up for a Retrobike ride too, in the spring, it will give me a goal to get my '95 'cone up and working!
Pete aka Woodsman
PS: If there was interest in a group ride Horsham way in the summer, I would be happy to lead, both St leonards and Tilgate forests can be included for a 25-30 mile ride, containing a high percentage of tucked away singletrack! ?