First, if the Diamond Dolls is something you'd like to do, do it for you. Not just because your crush is a baseball player. He may not be a crush forever, and then you'll be stuck with this commitment, make sense?
I would simply say to the other girls "I'd like to be paired up with Bob, we're already kind of friends, and I'm kinda shy." Or something like that. Chances are you're going to get a little razzle because you asked for him specifically, but ... Eh, just shrug it off hun. It's high school.
Also, I'd just start talking to him more ... Find out what he likes, if it's something that you like, or are interested in learning more of, emphasize on that.
You: "Hey, do you like pizza?"
You: "Cool, so do I"
You: "Whats your favorite topping"
You: "Ewww... Your gross!"
You might find that he has the personality of a pet rock, and you need to move on.