I think it's more a personality thing. There are plenty of quiet guys who are part of the group, but don't say much in group settings. In any group setting, male, female, or mixed, you're always going to have one or two people who dominate conversation. Admittedly, in a mixed group it will usually be a guy.
If you don't know these guys well yet, then sometimes it is prudent to hang back a bit at first. If you do know them pretty well, then just try saying what's on your mind about the topic. Don't worry about sounding stupid or saying the wrong thing. Just follow the conversation and make sure you're jumping in when appropriate. You know, make sure you're not cutting anybody off and you'll be fine. Think about the topic at hand and your thoughts on it instead of yourself and how awkward or shy you feel.