I looking for suggestions to improve how I interact in big groups (i.e. more than about 4-5 people).

I'm fairly reserved by nature, not a big talker, and have a job that involves me sitting and thinking at a desk all day by myself. I generally connect pretty well one-on-one, but as the number of people increases my ability to interact charismatically with them goes down.

This was rammed home to me recently in a bar with some friends, when one of the girls made a comment about how quiet I am.

I'm fairly switched into the social dynamics with others, I just can't seem to slot myself in quite right. What's frustating is that I know I can be lively and entertaining - sometimes things click and I am the life of the party. I know I can be that guy, I just don't seem to be able to do it consistently.

How can I consistently be in a fun, high energy state when in these sorts of social situations? Any advice appreciated.