Riverside can send an invite to the Studio on behalf of the Host. Invitation emails for Guests look like this:
Subject: Host Name invited you to their show
From: Riverside.fm <email@example.com>
Using a computer, click Go To Studio -> to open the Studio in your browser.
Using a mobile device, tap Go To Studio -> to open the Studio in the Riverside mobile app.
A Host or Producer might also send you a personal Studio invite link in a calendar invitation, email, or message.