As long as you have students in a group that you own or co-teach, you can view their usernames and change their passwords from the "Members" tab in the Group.
The Username is located right below the student's name. To change a student's password, click "Manage," then "Change Password."
Here's some more detailed info, and a screenshot:
- Go to the Group page, then click "Members".
- Locate the student in the list.
- The Username is listed below the student's name.
- To change a password, click "Manage" to the right of the student, then "Change Password."