Saint Genesius: I tried email and username, but both times I was told they don't exist.
Ice_Mage: Try the user's profile page. Add the username at the end
https://www.gog.com/u/ to get there. Click the
Invite to friends button there.
If the user has disabled the profile page, try asking this person to add you as a friend. You can go to the chat page at
https://www.gog.com/en/u/ (username goes here) /chat once again substituting the placeholder with the actual username.
If the user has disabled chat as well, then you seem to know this person's e-mail address, so make your request there. Alternatively, you can try entering
<username goes here> site:gog.com into a search engine, and replying to one of this user's forum posts that turns up.
Thank you for the response, but we have done all of those things. The problem is GOG is not recognizing the email address or the username. Yet I can go to his user profile.