Totally with PU-Technician here on this one... just throw in some small lines that don't even have to lead the conversation somewhere and you will seem active in it and not just that weird uncertain guy that just sits around! Don't overdo this though... If you blast out a line after every single sentence they say, well... that's even more weird!
Another thing you could try: Lead the conversation somewhere, where you feel at home! Meaning: Get people to talk about a topic where you've got lots to tell. like when people are talking about holiday and you have been on this cool travel lately, just start talking about it! Lines like 'Hey that reminds me of...' and 'Hey that's exactly like...' -- when i was in the states on this awesome trip last year (for example) -- really do help a lot! This will first of all secure your status in the group, secondly make you more alpha and thirdly, if you got a nice story to tell about like that travel i was using as an example it will DHV you!

Good luck!
- DimplomATA -