DW will track when the invite was sent; check the Invite Someone page and concomitant Your Complete List Of Invite Codes page. I don't recall if it tracks who you send an invite to, but it might. In any event, until they accept the invite code is active and you can send out the code to another person. I've had to do this, as friends procrastinate.

When they accept the invitation, you're notified, and they're listed in the page above.

The notifications don't stack; you only know you're own invitees.