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.
The Access tab holds the Account access card: 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. The same answer is on a badge next to their name at the top of the page, so you can see it without switching tabs.
They get an email with a link to choose their own password. It works for 7 days and can only be used once. Sending a new link cancels any earlier one. A link also stops working once they have signed in.