You can create free classes for everyone to book. Go to Products > Classes > +New class and create a new online or in-person activity. You can find more information on how to create classes here.

You can also manually add your clients to a class: if you already have created a class and want a client or a couple of clients to join for free, you can add them. Go to Schedule and find the class you want them to join, click on it and then you will see the Add client section, write their names and then select the correct user.