Create Group permission to grant view of all group content
As per their documentation "The permission layers explained" it should work like this:


  I want my site admins to be able to administer all groups without having to join them.
  
  Assign them a global role, e.g.: "Site admin", and then assign the "Administer group" >permission to the matching advanced outsider role in the group type configuration >section. Anyone with the "Site admin" role set to their account will be able to >administer all groups they are not a member of.