I have a 20+ group in my story (i know, wtf is wrong with me) but they all mean so much. At first, the group is filled with survivors of horrible experiments which have left them scarred and with incredible (but dangerous) abilities. They live in a place called Abditory which is a realm hidden to the rest of the world as they recover or hide. Eventually, over time you see how their friendships bond them to become a family and this family quickly grows as they find more people like them (refugees, outcasts, lost souls, you name it.)
Since they have all experienced pain, loss, judgment, and regret; they are able to help heal each other, they're there when each other fall. They forge trust and loyalty out of hopelessness and fear. The purpose of the group is not only to add something to the story (abusive relationship, self-confidence, divorced parents) which gives light to real-world problems in a land of outlandish characters but their personalities, even though similar, somehow vary hugely. I've got the responsible, deputy leader type, I have the musical, optimist one, another is a sarcastic, independent tech geek and a serious, quiet and rather shy character. The list goes on and it fascinating to see how these people interact with each other and the outside world. If you're struggling to find a purpose for a character, add that to the story. Make the character aware that they don't have a purpose within this group, how are they going to react? Are they going to joke about it? Will it turn them bitter?
Just let your imagination run free.