The Community doesn't have authentication - instead it uses your institution's authentication in Canvas. So it needs to see a cookie from your Canvas to follow the breadcrumb back to authenticate them. So first question is - is the user using a system that has been logged in to your school's Canvas before? And is that system storing cookies? Is the professor using incognito mode or similar?
@cliff_cunningha , @abunag is absolutely right. And, we recommend that your bereft user follow the instructions in How do I log into the Community with my Canvas Account? to create a Community profile.
Because this is a question about the (Jive-based) Canvas Community rather than the Canvas learning management system, we've moved it to the https://community.canvaslms.com/groups/community?sr=search&searchId=071c0bfe-67fd-44ed-8bb1-b7986339..., where we discuss all-things-Community.
Scott... silly question: How do I Direct Message you?
(I have another coworker who reports not being able to log into the Canvas Community. The error says that his ID is already in use.)
@cliff_cunningha , here's the link to the lesson with instructions for DM: How do I send direct messages to people in the Canvas Community? Also, we've done a bit of guide reconfiguration on the backend, so if you're not immediately able to view that lesson, the tl;dr is: Navigate to your Canvas Community Inbox, click on Send Message (envelope), and populate the To field with Scott's name.