Riverside can send an invite to the Studio on behalf of the Host. Invitation emails for Producers look like this:
Subject: Host Name invited you to their show
From: Riverside.fm <support@riverside.fm>
NOTE:
A Host or another Producer might also send you a personal Studio invite link in a calendar invitation, email, or message. The link looks like this: https://riverside.fm/studio/studio-name?token=0123tktk....