We at CTS have found that, for some reason, the Firefox browser does not like the invitation links from Builder and can lead to an annoying loop where you accept the invitation but does not open in your selected account.
As we continue to monitor this situation, it's advised for users to use a different browser to accept the invitation, either by changing your computer's default browser or by copy and pasting the invitation url into a different browsers address bar.
If you continue to have issues with this situation, please feel free to reach out to support and we'll be happy to help you.