Feeling like the third wheel b/w friend and her bf. Help?

My friend has been dating this guy for about a year and a half. We used to be best friends, but lately, I feel like we have been drifting apart because she is always hanging out with him. I like both of them alot. They are nice, funny, and just great people to be around. But I always feel like I am the third wheel when I hang out with them. They have conversations that I don't really belong in. They giggle and tickle each other and stuff. I just feel like I don't belong with them. We have a big dance coming up on Saturday. I am going to sit with my friend and her boyfriend, but I'm very worried that I'm not going to fit in and they will feel like I'm intruding. What do I do to help prevent this feeling?