Hi Jessica,
username,firstname,lastname,email are mandatory headers.
You will also need course1 (short course name) and enrolperiod1 (number of days from today). You won't need groups unless you're making changes to the group memberships.
You could go to [yourmoodleside]/admin/user/user_bulk.php and filter by students in the particular course. Download that selection and edit it down to the mandatory fields listed above. Then add the course1 and enrolperiod columns and fill them down.
Then upload your csv file to /admin/tool/uploaduser/index.php
Catherine