Can you use iMessage on a MacBook without owning an iPhone? If so, how?

If they go to Messages/Preferences/Accounts, does the account show there? Is it enabled?

Messages Troubleshooting - Basic

Might be a corrupt .plist.

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Preferences/com.apple.iChat.plist. Move the .plist to your desktop.

Restart the computer, open the application and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

If that doesn’t work, repeat the above but delete the com.apple.imagent.plist.

Thanks to leonie and Ralph Johns (UK)for some information contained in this.

You can actually use iMessage through the email account associated with your Apple ID. Try this article: Use Messages with your Mac - Apple Support