What's a Girl to Do?

I'm in need of some advice from trusted friends. I have one group of friends that we'll call group A. I love them dearly, BUT (there's always that huge but in the way) they don't care for many of my other friends. For the sake of simplicity we'll call these other friends group B. Group A often talks bad about B. This puts me in such an uncomfortable situation. I want to remain friends with group A, but I often feel uncomfortable and out of place when they start talking about someone(s) in group B. Many times I will just get rather quiet when they start in like this, but sometimes I will try to defend the person(s) in group B being attacked. I'm not sure how clear this is coming across.
I guess to attempt to sum this up - I feel torn. I don't want to lose group A because they mean a lot to me, even if they don't realize it. If I was ever put in a situation where I had to choose I would have to choose B. I need advice on what the most Christian way to handle the situation when next I'm put in it. What do I say, how do I act, etc.