Just put yourself out there. Although TopSorted is a complete arse for ditching his friends, it might help to take some of his advice and just become fitter; it will help your confidence no end, it'll be easier to mix more, soon enough you'll be planting the seeds. I've never been in a relationship with a girl i've haven't known for at least a week, most girls are too timid to just approach a guy, unless it's at a party and they're off their head. So just become more talkative, intresting, worked for me anyways.
As for this warhammer friends thing. I've been mates with a guy, who from the age of about 10, played lots of computer games and warhammer. When we hit upper school, he grew his hair, became a skater and became friends with that lot. Last year of upper school, he hit the bottom classes, became friends with the smokers, cut his hair and wore baggy white tees, and those baseball caps with the stickers on.
Before, he could never pull and never tried, now he's getting laid every weekend. It seems while your friends won't hold you back, being in a particular group will go a long way. So, why not become friends with everyone and just be yourself?