To find your group's registration link, visit the My Class page.
You will see all of your groups listed on the Class Information tab.
You will see the option to the right of each group:
Choose View Link to view the entire link.
This link can then be distributed to students in whatever way you choose (written on the whiteboard, emailed, etc).
When they click the link, they will be automatically enrolled in the group. This method will work even if a student already has an account; they will just need to sign into their account first.