It is sent automatically when your employer creates your account and looks like this:
If your invitation has gone missing, first check folders like spam, junk, trash, promotions, etc.
Probably the best way to find a missing email is by using your email provider’s search feature, if available.
If it’s nowhere to be found, you may try requesting a password reset email by clicking the “Having trouble logging in?” link on the login screen.
Otherwise, please contact Support:
phone: 855-438-8271