The main problem as I see it is infiltrators. The larger your group gets, the more attention it brings to itself the more you have to worry about "plants". Even if you have a dedicated group of Intell people "investigators" you will never know who might be passing info outside your group. That is why a lot of people are hesitant to commit to a large group. There has to be a way to verify a persons loyalty and comment to the cause. I for one do not have the answer to that question, I wish I did. Until we can verify a persons true heart and soul we will be confined to small groups.