I think there must be at least one more variable as to why a user cannot be seen. I can show you a case where two users have both their profiles set to 'organization' visibility (Who can see your profile?). One can be found, the other cannot.
You might want to check if the role of the user permits them to join groups.
Signed in members can post, follow updates, and more. New here? Register a free account.
Find useful guides, FAQs, and documents to help you navigate and make the most of Esri Community.