Newbie Mountain Biker

Hey all...I could use some encouragement. I have the opportunity to go mountain biking tomorrow, and I'm really on the fence about doing it. I've gone about 4 times in the past and have had a good time, but I've only ever gone with my boyfriend. He's a much stronger rider than I, but he's patient and helps teach me new things. Tomorrow would be him and four of his co-workers (everyone has the day off for NYE)...did I mention that he works at a bike shop? So everyone will be a stronger rider than me, and I'm worried about embarrassing myself by being heavy/slow/unskilled. However, I want to find the courage to do it because I know it would be phenomenal exercise.

Has anyone had any experience being the newbie in a group of skilled people, especially with regards to some physical activity? How did you get past your shyness/embarrassment? Help!