Group friendships are sets of loosely knit friends. They require the presence of all group members for a get-together. Some members have a history of not clicking... and the meeting of such pairs would raise eyebrows. The main thing uniting a group friendship is that all are friends by connection.
A friend of mine who I have grown to love and care for was once part of a "group" I was in. When we told members of the group we were hanging out one day, we were met with shock. Even internally, it's understood that group members are not all friends.
But everyone maintains the status of mandatory invite. Exclusion is not acceptable, for somehow the fear of being left out encouraged a group of all-inclusiveness.
I have grown to disfavor group friendships for the unnecessary and uncomfortable meetings, the lack of real unity, and the maintainance of friendships for convenience's sake (who wants to be the person who has beef with a "secured" group member?). I have found such friendships to be unfulfilling and even unhealthy at times.
more: comfy friends, etc