To invite you to their workspace, they should go to Settings > Team > +Invite, enter your email address, select your role (User or Admin), and click Send Invite.
If they're getting error messages, they should try clearing their browser cache and refreshing the page.
For troubleshooting the specific error, it would help to know:
** What the exact error message says
** A screenshot of the error
* Which page they're seeing this on
Can you share what error message they're seeing?