To book Customers into a Class Reservation (either a once off, or a timetabled one), you must first create the Class Reservation 

(see How do I create a (once off) Class Reservation?  or How do I create a (Timetable) Class Reservation?)

You can book a Customer onto a Class Reservation by tapping directly onto the Reservation tile in the Schedule or Timetable

Scroll down to Customers and tap on the arrow

Tap the + icon and search for your customer by typing his/her name into the search bar

Tap on the Customer, then tap Save

OR

Enter New Customer details in the Select Customer popup box and tap Save

The Class tile on the schedule will now show an increased number of attendees