Send them a link to set their own password — no need to contact support.
Click "Instructors" in the sidebar, then click the instructor. Anyone who can't sign in is marked on the list with an amber badge, so you can spot them without being told.
On the right you'll see whether they have ever signed in and whether they have a password. "No password set" means they were invited with a link and never chose one — they can't get back in on their own.
They get an email with a link to choose their own password. It works for 24 hours and can only be used once. Sending a new link cancels any earlier one.