As a teacher, if you have the permission to manage your classes, you can add teachers and students to any class where you are already set as a teacher.
Go to the Classes page and click on 'manage' next to the class you want to organise. In the popup window, go to the 'Members' tab.
If you created this class, you will already be the class's teacher. You can add more members by clicking on "Add a user to this class". This button will be greyed out if you do not have the correct permissions.
Click on the "Add a user to this class" button and start typing the name of the user you wish to add. This will search all of the users in your school and display their name under the input box.
Click on their name, and they will be added as a student to the class. (You can change their role by selecting the appropriate radio button). The student will now show up on the Teacher's Dashboard.