To be able to configure the attendance fees to activities we will go to:
Customers → General Rates → Group classes → Activity Fees
From here we will access the next window, where we will see a list of quotas, where we can add more with "New", edit any of the existing ones with 
or even remove some with 
.
If we click on the button to edit some of them will open the following window:
From here, in the General section of the menu on the left, we can choose which activities are included:
While in the Prices section, we can have different rates, if we want individual or collective classes and what price has each one of them.
