If you invite your mutuals (aka people you've partied with before!) directly through Partiful, you'll be able to manually change their RSVP! If your guest was invited through contact sync or hasn't already signed up for Partiful, you cannot modify their response.
To change your guests' RSVPs, click on your guest list, and then click on the drop down menu next to your guest's name. The guest will not be notified.
To invite guests directly through Partiful, you can either sync your contacts in our app or invite your Partiful mutuals (aka people you've partied with before!). You can invite mutuals by filtering by past events that you've attended or hosted - click "Invite" on your event page, and then click the filter button in the top right corner to see past events.