If your experience is like mine there will be many more important considerations in finding the right group than pace. In CA, MO and here in WA I have found large group riding to be a PIMA (Pain In My Asz). There are always a few people who demand on asserting cyclist's rights on the road. I know the problem is 99% mine, but I see too many riders that I'm not compatible with. Be safe.

edit: I believe in cyclist's rights on the road and even participated in a couple of Critical Mass rides in San Francisco, so please no flames about my opinion of large cycling groups. I simply choose to not participate in group rides, with the exception of scheduled events where the driving public is well aware that groups of cyclists will be on the road.