Overview
This article goes over how to add a membership to a customer from the Admin side.
How
Go to the customer's profile page.
On the upper right hand side of the page, the list of memberships purchased will appear. If they have never purchased a specific membership, you can press the "Buy" button to display a list of available memberships.
Choose the membership they need for the program they are signing up for. This will automatically place it into the shopping cart.
Proceed to Check-out as usual.