I'm assuming that this is accomplished by deleting the default 'Members' role. Here are some steps that I did to replicate it:
Creating the group...
  • Create an invite only group
  • Have private visibility
  • Set the new group roles as "Managed Invite-Only"
-Finish creating the group
After the group is created...
  • Delete All the roles that you can
  • Create a new role not named 'Member'
  • Invite someone and have them join
After the person joins, Try to do anything to the user like adding roles.