Are these users showing up as a member of the Site context instead of their proper org? A participant cache corruption could cause certain attributes like the email address to disappear, and the user's context to show up as Site instead of their org.
Open up the Participant Administrator from the Site->Utilities page. Search for one of the users seeing this issue and add them to the table. Then, right click on the user and select Remove from Cache. This will remove the user's corrupted cache entry. Then, view the user's information to re-add the user to the cache. The email address and proper org should now show up.