The purple button in the action column allows you to add users to your Class/Group.
A pop up window will appear and you can filter the users by year level or user type.
Once you have selected all the users you want in your Class/Group, press Add and the new users will be added to the class to start receiving work you assign to them from then on.
Option 2:
The second option is to allow the students to join themselves. Once you have created the empty class you can invite your students to use the 'Join Class/Group' tag under 'ZAPP PARTICIPATION' to join your class. They will see a list of teachers in the school. When they select your name they will see a list of your classes. Once they have selected the correct class they can join the class. Once the students have joined the class you can lock the class so that no one else can join.