group interaction is much more difficult for me too - it's easier if it's structured, like in a drama workshop or a course/class/seminar on something interesting where debate is encouraged and moderated...
it depends if you know people well or not, it helps to get to know them better and what they like, wish, want, admire... what you all have in common etc. then it's easier to present your ideas in a way that would be interesting for them - if they like films, you speak about films, etc. If everyone in the group is interested in different things, it may be a bit more difficult, usually people do share at least some interests though, so you can talk about that, or new things you've seen/heard about etc.
don't get discouraged if people speak on top of each other, or don't hear you, usually there's a lull after that, and you can get some words in.. you can ask again, or connect it with a different sentence/topic or a few..
if anyone says how quiet you are, it means they've run out of things to say and would like to hear what you have to say - maybe they're intrigued by your mysterious eyes or just interested to hear your thoughts! (or just don't know what to say and want to continue the conversation!)
you can joke back or have a few 'ready replies' - things that would be cool to talk about, or new things you heard on the radio, 'Hey, did you hear/see...?' Many people like to hear new stuff, especially if it's interesting, or discuss it..