Interestingly enough Kate, there are a few different groups of local womens' training rides. I might cross paths, pass, or be passed by them. We say hello, but I think if they've gone to the trouble to organize their ride as such, it would be just rude to try to jump in with their group...yep, minimal social skills are important.
ovoleg, if it were me you were behind, I'd probably pull out and slow up a bit, and say hello. Having done that, you'd probably ask if I minded if you hung on the back, and I'd say no problem. Hang on as long as you can.