Yes there does appear to be group Kamma. I have postulated, and that's just my opinion, reflection, that perhaps all of us, if past lives are true, that we may have been 'hanging out' together, and that past life connection has brought us together again. The same with good Kamma and the same also with negative Kamma. It could also be possible that this force was working and that people who had to die at a certain time due to their past Kamma came together at that time in that area.
In some of the Jataka stories, supposedly from the Buddha's past lives when he was a Bodhisattva, it talks a lot about some of his disciples in that life when he was the Buddha, being connected with developing the Paramis with him in past lives. And how this connection brought them together again in that current life. Sometimes it's very difficult to understand the connections we make with people that seem almost immediate. For Steve and me it was immediate, on meeting there was a connection that could seem unexplainable. I had someone say to me at the beginning of the retreat, "I think I have seen you before," yet they hadn't met me before.
Many times I have these feelings about certain people as well, that I know them. I don't believe anything happens by chance. So it could be that this is a way of how to explain it - past life connections, and this may explain why there's group Kamma. That certain people were also together in past lives and then they came together again in this life at certain times. Since I'm not a Buddha, I can't actually say from my own experience. I don't know whether Steve has mentioned already that the Buddha in the scriptures said that if you try to figure out all the aspects of Kamma that you'll probably go mad. That only a Buddha can really know this.