You have the ability to send a message to multiple guests at once through Punchbowl. First, tap the Manage Guests
button for your event.
On the next page, there will be a Message Guests
option towards the top.
This will give you the option to enter a message to send to guests. At the bottom of the page, you can choose if you want to send to guests who have RSVP'd Yes, guests who have RSVP'd No, guests who have not responded, or guests who are undecided. You can also check all four boxes to send to all guests.