https://mnsite.ims.mnscu.edu/shared/_instructor_and_coursedesigner_help/online_help.htm

Here are the snapshots:

Go

Communications

Classlist

D2l classlist

Click Add Partcipants and scroll down

Sellect Add existing users

1. type name in the “search for” box
2. click hour glass icon to search
3. after finding the name, check the box in front of it
4. assign the correct role
5. click bottom left “Enroll Selected users