If your users already have existing accounts in the LMS, you can easily enroll them in the course by selecting their names from the list of users. 

1. Select Select from Existing List from the list of enrollment options. To view your enrollment options, read this article: https://support.castlms.com/help/viewing-enrollment-options)

2. Select the users you want to enroll in the course by ticking the checkboxes across their names. You can also tick the boxes of a group to select all the users in it. (You can unveil the list of users for each group by click on the group name.)

3. Click on the Update User Enrollments button once you're done. 

WARNING: Do not untick the boxes across users whose boxes are already ticked. This would unenroll them from the course and delete all their progress.