Inviting external users to become managed users

Hi, I have 4 starter accounts that have been set up by team members, that are not currently managed by my admin account. When I try to invite the external user accounts to become managed users the error message "There was an error inviting username@domain.com to join your enterprise. You can contact this user directly to log in to their account." is returned. Why am I unable to invite the external users? I have also asked one of the external users to log into their account to see if there is a way to request to become a managed user, but there is nothing that standouts as providing that functionality. Thanks for any assistance offered.