To do this, click on "Home" on the left side in FitogramPro and click on the event for which the attendance is to be marked. For the booked-in customers, you will find two thumbs next to their name - one for present, one for absent.
If a customer books in online, they will not be marked as "Present" or "Absent", as it is not yet clear whether they are actually participating in the course.
If you book in the customer, they will immediately be marked as "Present", because then they are most likely standing in front of you and they have booked the course.
If one of your participants does not come to a booked lesson, you can either mark them as "Absent" with the "thumbs down" or cancel them directly from the course via the "X".