@Debbie M I didn't get the impression that @Naomi M was talking about "lookups" as in cross-referenced fields, but I could be wrong.
@Naomi M I would add that if you really just want a big button that says "RSVP", you don't need to show the Lead their own details to update the Lead record. If they click a link in a Marketo e-mail, their cookie will be associated with their Lead record automatically, so they will be updating the database. (You can actually give them a form that doesn't have any fields at all, just a big Submit button renamed to RSVP. Then look for a Filled Out Form activity.)
If this doesn't apply, I need to know more details about your goals.