It sounds like you're doing just great, although I'd emphasize that their is no wrong or right speed. As for joining a group ride, it really depends on the group and what kind of ride it is. Some are fast training rides where the group rides in a tight "paceline." That's probably not quite what you're looking for at this point. Other groups take a slightly more relaxed pace and ride as a group, but not necessarily in a paceline. Others are strictly social. I'd also note that some groups have a "no-drop" policy, while others expect you to fend for yourself. My local club, along with some area bike shops, offers a bunch of different types of group rides. There's something for everyone.
Do you have a local club? I'd find them on the Web if you can and see what information is available.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher