Problem: Site Owners with full control couldn’t access or edit members of a group.
Root cause: By default, only SharePoint group owners can add/remove group members, even though you have full control access rights to the site – This is by design!
Solution: Change the group owner! You can either add a person to the “Group Owner” or set the group owner to the “Site Owners” group (recommended) so that any member of the site owners group can add-remove members to the group.
Alternatively, You can change group settings to allow members of the group to change group membership. (but not recommended!). Under the group settings, set “Who can edit the membership of the group?” to “Group Members”.
You do not have permission to view the membership of the group
In some cases, users may encounter a “You do not have permission to view the membership of the group.” message on trying to access the group. To fix this issue, under group settings, set “Who can view the membership of the group?” to “Everyone.”