#open works on a double opt-in system. In order to send messages, you first need to match with another member: if you like someone’s profile and they like your profile back (or vice versa), you'll become matched, and both of you will be notified that you have a new match available.
Once you're matched, you can send a message:
-Navigate to Messages (top right)
-Tap the thumbnail of the profile you wish to chat with in New Connections
-Type your message in the new chat thread and hit send!