As long as you have students in a class/group that you own or co-teach, you can view their usernames and change their passwords from the Members page in the class/group.
The username is located right below the students' name. To change a students' password, click on the more icon next to their name, then Change Password.
Here's some more detailed info, and a screenshot:
- Go to the class/group page, then click Members in the left panel.
- Locate the student in the list. The username is listed below the student's name.
- To change a password, click the more icon next to the students' name.
- Click on Change Password.