Each time I click the link in my email it says the link has expired. I send them a new one after that, and they click the new one and still get the expired message.

Please re-send the invitation and clear your browser cookies and/or accept it using an incognito window.

When the invite expires immediately like this, it's typically because you are accepting the invite from a browser where you are already signed into the Sentry organization on another user's profile (like a server account, for example).

Occasionally, certain browser settings/extensions have been known to cause issues with accepting invitations, in which case using an incognito window can often resolve those issues.