Booking a customer into an event (class) using their membership can be achieved:
from the Point of Sale; or
from the event's page
For POS,
either click Home > Point of Sale or use the POS shortcut in the top right.
Choose the customer > select the event > click Add to Cart
In the Payment section, select Membership > choose a membership > click Book with Membership
To book from the event page,
click Classes > Schedule > select a class
In the 'Add customer into this class' section, search for the customer > select a membership below
The customer will be booked in automatically