Under Courses, choose Create share link for a course, then copy it. Each link has its own random address, like /course/k3Xq9TzL2mPw7RbA, so it can't be guessed.
What people see
Anyone with the link can see the course's title, summary and length without signing in. To take it, they sign in with Discord. Their progress then appears under Team progress.
A shared link doesn't make the course required. It just lets people open it.
In Discord
Paste the link into Discord, and it shows a preview with the course's title, summary and length.
Turning a link off
Choose Turn off link and it stops working straight away. Progress people have already made stays. If you create a new link later, it gets a new address.