You can accept friend requests in the friends tab. You'll see a notification when you have pending friend requests.
When a friend attempts to connect to your PC, you may see a Windows Action Center Message (if you have that enabled) prompting you to accept the connection attempt. Once you accept the request from within the Parsec app, you can pass the controls to the user by clicking their avatar at the bottom of the app. 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.