Providing each student with their own unique student ID is a secure way for them to join your private room. The ID's are created by teachers when adding a roster to the room.
To view ID's for students in the currently active classroom, click on the Roster menu at the top-right while logged into the app:
Click Show Student IDs to show
The ID numbers will be shown just below each student's name
The hand raise feature is also shown in this menu. Learn more about it in this link.