Hi Steve,
Thanks for reaching out. Unfortunately, there is no way to restrict invitations to a specific email address. We will take this feature request under advisement.
One alternative you could consider would be to use the SSO feature available on the Organizational plan. This plan gives account administrators the ability to configure their accounts to require users to authenticate using their institution's SSO provider such as Active Directory or Shibboleth. When registering for an RStudio Cloud account using SSO, the email address will be determined by the SSO provider.
Best,
-Andy