When you go into the Users option under your Account drop-down menu, you'll be presented with a list of your Users. Once you select your specific User you'll be presented with many options, including the Update and Resend Invitation button down at the bottom of the principal page.
This DOES NOT apply when the user has logged in prior and has been active in the past but has locked themselves out of their account. For those instructions, please see the tutorial on either unlocking a user as an Administrator or resetting your password.
Note: For security reasons, invitations expire after 14 days. If the User has not logged in by then, please contact our Support to have our team resend the invitation.