So I'm in an all guys high school/college and currently taking part in the school production. So some girls from a school nearby come over and we dance/act/sing/etc for a few hours after school.

The problem I'm currently having is that during these rehearsals I am always the quiet one.. WHY!? When I'm with all my guy friends I partake at least a bit and have some witty comments on hand. However when these girls come over and guys who I hardly know I always feel like I'm just TRYING to laugh at everything to feel part of the convo but not actually putting in any input.

I guess I feel a bit scared to say anything incase no one laughs? or no one acknowledges it? But to be honest my mind seems to go completely blank and I don't know what to say!

How do I counter this and start engaging in the conversations, making the others laugh etc, feeling dominant?

First you need to realise that having a good conversation and telling jokes that make people laugh, isn't about trying to be funny or interesting, if you are trying hard to be funny and interesting, you'll probably end up looking awkward and not fit in. None of the other people are truly all that worried about whether the other people they are talking to are laughing or not.

The best conversationalists talk about what THEY are interested in and what THEY find funny, not because they are hoping to impress others and fit in, but because they are doing so for their own amusement and enjoyment. The moment you start trying to impress those other people it is going to go down hill, so you don't worry about what they think and you just be confident that you are talking about something you are happy talking about because you know that it's cool shit. If you like it and they can see it in you, then they'll be interested too.

As far as making people laugh, just make sure that you don't laugh at your own jokes before they do. Watch a lot of HBO comedy shows and whatever else you can get your hands on (live standup is the best IMO) and watch how the comedians tailor their reactions to how the crowd reacts in order to achieve the best results. Watch how the comedians will wait just a fraction of a second after everyone else starts laughing, then they laugh too. I highly recommend checking out Dane Cook, Chris Rock, Joe Rogan and any other animated comedians that you can think of because they will also teach you how to use your body language to enhance your jokes.

There is a chance that you just don't fit in with those types of people. As you don't seem to be able to connect with them that well. This may be due to not very good conversation skills tho.