I have the same problem, that is the member registered and the display name (Full name) is overwritten by username. And also I cannot change it using edit profiles (I typed the new one but it is still overwritten. @r-a-y do you have any other solution? and for edit profile?
Moreover, if a group is set public by admin and less than 20 members (let say 10 members), it becomes hidden if 1 more member join group (total 11 members), that means the way to change from public to hidden is ok. But changing from hidden to public may be required more, can anyone help me?