Private Messaging allows users to send messages to another user in the app.
If the event organizers have enabled this feature for your app, you can turn it on/off by navigating to your profile, tapping your name from the app menu, and then tapping the app overflow menu. From there, tap on “Settings”, and toggle the "Allow Private Messaging" option to turn private messaging on/off.
If another attendee has this feature on, you should be able to see "Send Message" beside their profile picture when you tap on their profiles. Tapping on "Send Message" will allow you to send an email to the address they have used to register for the app.