The work around that I use is to simply delete the record from the wp_signups table:
delete from wp_signups where user_login = 'username';
Where username is the username you set when you tried to invite the user.
If you don't have direct database access, I would recommend ticking the box that says "Skip Confirmation Email." After that, you can manually set the password and send it through a secure service like Lockify, or you can send instructions to the user to use the password recovery system to set their own password.