If you're on Windows 8.1+ or macOS 10.15+, you can share a link so that people can join your computer. Anyone with the link can open it, or paste it into the join field in the bottom-right of Parsec.
Desktop mode (Computers tab)
Click "Share" in the Computers section in Parsec to create one, then copy the link and send it to someone. The link can be invalidated at any time by clicking the X button. When people use this link, you will get a connection request in Parsec, which you'll want to accept to start streaming.
If you're hosting a game in Arcade, the link is given once you start hosting, in the game's icon at the bottom. To invalidate the link, stop hosting by closing the game or clicking "Stop Hosting" in the in-game overlay. When people use this link, they will automatically join the game and be able to play.