Just ask "do you mind if I join?".

I'm a solo rider and if I find a group going my pace (or a bit faster) and in my direction., i ask and have never been told no. Depending on how long i'm with them, i'll work my way forward and do a pull. When they go a different way I always say "dropping off" and a rally loud THANKS.

There's a difference between joning in and being a wheel sucker.