Can't add family member to OnStar Guardian who was previously removed from My Family

My husband was in my OnStar Family previously.  I recently tried to add him back to the family, but when I send the invitation he receives the text and when he clicks on it, but it just opens the app and does not show him where to enter the code I sent him.  He verified that he's logged in, and that his login is the same username that was used previously when he was in the family.  How do I add a user that was previously in the family but was removed?