You can accept friend requests either by clicking on the Windows Toast Notification or through the hosting sidebar on the left of the Parsec Application.
When a friend attempts to connect to your PC, you'll may see a Windows Action Center Message (if you have that enabled) prompting you to accept the connection attempt. When that message prompt appears, you can click to accept, or just visit the hosting tab in Parsec. Once that occurs, you can pass the controls to the user in the sidebar. You have the choice of giving your friend controller access, keyboard access, or mouse or a combination of the three.
If you do not have Windows Action Center activated, you will not receive this message. You can turn it on in the Notification Center of your computer's control panel.