If we have a group of client's with discount on some prices, such as children, unemployed people etc. we can add a person to this group, accessing the customer's file.

In the general section on the left menu, we have a field that says "Groups" and a button that says "Add group", press the button and choose on the popup window the group of customers where you want to add this customer.

You can select a period of time when the customer would belong to the group, optionally, and once you press Update, on the client's file will appear the group to where he/she belongs under the "Add Groups" button. 

A customer can belong to more than one group.