If you need to update the person's email address or any other data about the person (spelling of name, phone, etc.) do that first by clicking on EDIT next to the person's email address.
After you have completed updating the person's information, click on UPDATE at the bottom of the screen.
Next, click on the person's email address you wish to resend the welcome email to.
On the next screen, click on RE-SEND WELCOME E-MAIL