We don't have a specific option to do this at the moment, so please check back with us. For now, you can achieve this through a work around that works well for many users.
On your desired "RSVP by" date, simply log into your account and click "More" beside your invitation and choose "Add Guests."
Under the "Guest Options" settings bar on the right, use the "Set max capacity for guest list" option to enter the current amount of "Yes" RSVPs that you have received. This will prevent any other guests from RSVPing "Yes". Guests will receive a message to reach out to the host if they attempt to RSVP "Yes".
Once you have made the changes, click on "Update" in the top right corner.