One of my favorite group rides used to have a rider who regularly showed up riding fixed (I rode my regular road/race bike). The ride was a mid-pace fairly relaxed one, and he kept right up and fit in fine. He did run a front brake though. While I try hard to stay out of the brake politics, I have to say I would have been uncomfortable in close roadie quarters with a brakeless rider (and to be fair there are a lot of regular roadies on group rides that I don't like going near), and I doubt I am the only one who feels this way. But if you are a good rider, can keep up, and use a brake, you should be able to find some road group rides to get in on.