Join a group discussion indirectly

Join a group that includes someone you already know.
Chances are, your friend will invite you into the conversation or someone else in the group will ask your friend to introduce you.

Participate as if you were in the group all along.
This can feel awkward at first but people will quickly accept your presence, especially if you are in a context where people are moving in and out of groups all the time.

Mention something you overheard.
For example, say something like, “Wait, are you guys talking about the new Switchfoot album? I like their older stuff – do you think I should get the new album?”