How did the group therapy guys know to come and save Charlie?

I think the assumption is that the group was contacted by the one member Charlie calls just before the deal. It's a nice scene, but doesn't really support anything else. So I think it was inserted to lend credence to the group coming the the rescue.

One of the things I like about the ending is that the whole group gets saved. They all overcome their fears and restore their confidence by saving Charlie. Much better than leaving them on the sidelines at the conclusion.