If you've sent an Invite to a Client previously but they haven't yet to registered their account, you can send them another invitation quite easily.

From your Client Listing and you'll see a 'Resend Invite' button on any Client you've already invited to the system but they haven't yet accepted.  

When you click 'Resend Invite', you can use the standard messaging or add language of your own. Just be sure to keep that [magicLink] smart tag intact!

Click 'Send Invitation' and your invitation will be on its way.