A lot of this has to do with your own self-confidence issues. If you clam up around people you respect and are friendly with, it’s probably coming from a feeling that you’re not as smart or funny as they are. You’re giving them a power you feel you don’t have and are withdrawing into yourself -- even though you probably have a lot more to offer to the dynamic than you’re giving yourself credit for.
True, some people tend to be stronger personalities than others in group settings. But you need to ask yourself why you’re allowing yourself to be diminished. You can admire someone without being overpowered by them.
If someone has a particularly strong personality, there are strategies you can employ to make them take notice and respect your role in the group:
Then, and probably most important, relax. You are who you are! Be comfortable with that and others will be too.