Hey Crystal and happy Friday!
Great question. I think the easiest way to do it would be to complete the RSVP form on the front end on their behalf. However, I see you have a login requirement, so that might not be the best route.
Another way would be to try using the User Switching plugin. If you are a WordPress administrator on the site, then you can access their user profile in the WordPress admin, switch to being logged in as them, and complete the form.
Do you think that would work? Let me know!
Cheers,
Geoff