Normally, when you invite someone, they get an email and are asked to set a password.

If they can find this email, they will be able to login.

If they try to create an account with the same email, they will be told there is already an account with that email.

The invited user can just login using their own account and start working with you on any projects. If they have forgotten their password all they need to do is click to 'reset password' - with the email they were invited with - and they will be asked to set a new password.