This is incredibly late and I'm aware of that, I didn't expect to see anything when I checked the group.
I'm not entirely sure the founder is still active here, and they're the only one able to manage members. I don't think being a Contributor will stop you from being another group's co-founder? I think what stops that is being the co-founder of two other groups...maybe.
I hope you've figured this out by now, and if not, hope this helps and I'm sorry for the problem! DX