I've been on both sides of this. On the "outside" side, I have noticed people who were popular, and even though they were sweet and friendly and there was nothing WRONG with anything they were doing, I felt annoyed with them. I really had to examine myself to see if the problem was them, or that I was feeling left out or jealous. I hate to admit that for me, it was simple resentment. These girls had never done anything to harm me, and probably had no idea that I was feeling like an outsider.
I've been on the popular side too, and felt a burden to make sure that no one else around me felt left out (because I knew how it felt). I would expend great energy trying to bring others into the circle. I would sometimes be frustrated that others in my group didn't see the importance of doing that. I'm in a bit of a "close group of friends" right now at church... and I feel guilty about it even though I don't believe it's wrong, in itself. God gives us close friends sometimes for a reason, and I am so thankful for my close friends. I've done all I can do to draw more people in and make them welcome, but there's only so much that we can do. For one reason or another, many people just can't participate in activities or meetings that our group has outside of church. While I feel terrible that anyone might feel left out, I don't know what the solution would be to fix this.
There are always going to be, in any group, people who are confident, who draw others toward themselves. People with magnetic personalities. They can't necessarily help it. (And they will naturally be more talkative with those that they feel like they know better.) They can use it for good though. I don't really know the situation at your church, but the best thing you can do is find some ways that they might be able to help other people feel included and welcomed, and perhaps encourage them to do those things. It sounds like they are well-intentioned and just need to be made aware of the problem.