Add learners to a course

Hello,

I am hosting a course for a client using his Squarespace, and I cannot figure out how to add learners to a course and help then navigate back to the course on return visits.

They are paying via invoice, so I don't need a digital product purchase flow at all.

There's no courses topic tag available, so I have no idea what the visibility on this will be like in Forums. 

Thanks!