Hi Erin,
While I believe you can't create a unique url that you'd be able to access as such, using the data tag
{[E-RSVP YES URL]} behind your button should achieve the same purpose.
If you want to make extra sure nobody but the people on your invitation list are able to register, you can also enable the setting "Only those on invitation lists" in the Privacy Settings of your Default Registration Path.
Best, ------------------------------
Béline FALZON
Conference Program Specialist II
California Teachers Association
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