Chris phoned. He still had the same error when he tried to access the site:
I decided to remove the guest account then add it back in as follows:
1. Tried to delete the guest user account from Active users, but it timed out.
2. Removed the guest user account from the SharePoint member group in SharePoint Admin Center>Active Sites>GCF Investment Committee ... Members
3. ... then deleted the guest user account from Active users .
4. Created the user account again in Admin Center>Users>Guest Users ... which sent an invite.
5. Returned to SharePoint Admin Center>Active Sites>GCF Investment Committee ... and a added him as a member.
Phoned Chris, had him find the invite whish had him sign in and get an activation code. That took him to a personal user page.
Asked him to go to the URL for the site. That worked. He could now get in.
Had him close the browser and reopen it and enter the site again.