One person's idea of a beginner ride will be different than someone else's.
Regardless, my number one advice to the OP is to introduce yourself to the group beforehand, let them know that you are new, and ask for advice, what is exepcted, etc.
Also, know the route and be prepared to fix your own flats, etc.
Wise words. Its been just 3 weeks or so since my first group "beginner" ride and at that particular ride was 20mph+.
I would show up prepared to ride as hard as you ever have. And if it turns out that the group is slower so be it.