Can't add Customer users to Chatter groups

I'm trying to add a customer user(profile: Customer Community Plus User) to a chatter group(private - allow customers is checked).   I added all possible permissions to the customer profile and checked all Chatter Settings(in setup).  When I try adding users of this profile this error pops up: 

  Please note that I'm using the "New Group Member" button. When using the Manage Members the Users of this profile don't appear. @* Chatter Adoption & Best Practices *