Do my guests need to have the Mac desktop app in order to record?

No, your guests do not need to download the MacOS desktop app to record with you in your Riverside studio.

You and your guests can all join your Riverside studio from different platforms, including from a computer browser, mobile app, or the MacOS desktop app.

For example, you can join the Riverside Studio from the MacOS desktop app, while your guest records from their computer browser.