When guest users who are already authenticated to Tableau Server attempt to open a dashboard on a Site with enabled guest access, which is not the Site they were previously logged into, they are being prompted for Tableau Sign in. When signing in, the following error message will occur:
You're not a member of this site or this site does not exist. For help, contact your Tableau Server administrator.
This happens when the user is also a member of another Site in Tableau Server and they access their account/Site before using the embedded link or Share URL for the guest Site view.