Hi Abby,
Thanks for reaching out! What a great question, let me help you with that.
I believe our free Event Tickets plugin would be a great solution for that.
Event Tickets allows you to collect RSVPs for events. That functionality could be used for collecting interest as well.
Here is our demo site, on this page you can check that out in action:
The section called “RSVP” gives you that functionality. If you increase the number, then a couple of fields will appear where visitors need to submit their names and email, and signal whether they are interested or not (Going / Not going. This label can be changed with a tiny piece of code to “Interested / Not interested”.)
The section below called “Tickets” is done by our Event Tickets Plus plugin, in case you are looking into selling tickets in the future.
Let me know if you have any further questions.
Cheers,
Andras