Oprah is well aware of the social leverage that her celebrity has. Maybe part of the reason for her declining is that she knows that if she were to accept membership in one group, there could be a stampede of ladies wanting to join that group since Oprah is a member, and this could potentially be at the expense of other groups on campus.

Another thing that she might not like is that GLO's, by their nature, do exclude people. This might not be something that she is comfortable with.