You can add new students to a class by generating a game code for the quiz. The students can join the class by joining the quiz using the generated game code.
For a Live game, click 'Still need a game code?' link from the Live dashboard as shown in the image below.
For a Homework game, Click the 'Click here' link from the reports page to generate a game code.
Now, click 'CREATE ANYWAY' button to generate a game code.
Note: You can also add players by going to the "Classes" tab and selecting the class and clicking "Add students". This will give you an invite link that can be shared with students.