Providing each student their own unique student ID is secure method for them to join your private room. The IDs are created by teachers when creating a roster in the room.
To view IDs 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 or hide them
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.