In order to share a course with clients, you must first create a group then assign the course to it.
Step 1: Create a new group. Go to 'My Groups' then click on 'Create New Group'
Step 2: Under the Course tab, select a course to assign to the group
Step 3: Once you finish setting up the group, click the "Copy Link" button to grab a client invitation link. This is the link that you will share with your client.
Note: You cannot share the course directly. It must be part of a group.