To indicate to a client as an assistant of an activity we must access the table of reservations and the specific activity in which you want to see the attendees to an activity.

Click on the activity in question and we will see the following window:

Click on Add Players to indicate a client as an attendee to the activity. 

Once we have pressed, we will see the list of clients and click on the icon that is indicated in the following capture:

Once added the client will be in the list of participants.

To indicate if any of the participants that we have added to the activity did not attend, we point it out in the Asistance dropdown:

In the drop-down Assistance we see if the customer has attended the class or has missed.