There are a couple of other questions on this, but neither covers my situation.
A user that was logged in with an Atlassian ID but unknown to my site found that he couldn't access it unless he logged out.
He was not a member of any group, because he wasn't registered as a user on my site. In this case, he should have been treated as anonymous because his authentication as an Atlassian ID user is irrelevant to me since I don't know him.
Other questioners have been told that the users group needed to be given "Use Confluence" permission, but this user is not a member of that group. In any case, I can't give that group the proper permissions.