To message to a group of guests on Partiful, you can send a Text Blast. Partiful lets you send a Text Blast to guests who have RSVP’d or were invited directly through the app or on web. Hosts can send up to 10 text blasts per event.
Just head to your event page - on mobile, you'll see the Text Blast button at the bottom of your screen, and on web it will be on the righthand toolbar.
Hit New Blast to create your custom message, then select which guest statuses you'd like to send this to.
When your message is ready to go, just hit Send!