You can do this from Point of Sale or a Class Details page.
From POS, you'll see the following message if the class has the waitlist enabled.
Click 'Add to cart' and you'll see a waitlist tag attached. Next choose payment method below, and
Next, choose payment method below for the would be charge if they end up making into class.
From a class page
In the class details page, you'll see the following message if the class is full.
By adding someone in, you'll be presented with this pop up that gives you two options: override the class capacity or add to the waitlist. Choose the wait list option.