Friend requests can be sent and accepted by using the Friends icon in the sidebar. Click Add Friend to send a request and search for the name and ID they use in Parsec, which they can see at the top-right in their app. The other person can then click View Friend Requests to accept it.
Once you become friends with someone, you can request to join each other's computers in the Computers section. The host should see a Windows notification and a section in the interface. They can accept the connection via the interface or by pressing CTRL+F1.
The Windows notification might not show up if it's disabled or in Focus Assist mode. In that case, just accept via Parsec's interface. Once your friend joins, you click them to grant input permissions.