To turn on hosting, you must use an (Windows 8.1+ or macOS 10.15+), and install the native application instead using of the web client. Other operating systems and the web client do not support hosting. If hosting is already enabled, you and your friends will see your computer in the Computers tab, and your own computer will show with a "Share" button.
You can switch whether hosting is enabled with the "Hosting Enabled" option in the Parsec settings' Host tab. Hosting should already be turned on by default for Windows 8.1+ machines, but is disabled by default on macOS. If you're interested in hosting on macOS, learn more about the macOS hosting preview.