You can message a user that checked in at a station by tapping on their check-in.

  ** If a check-in is posted by "A PlugSharer" it means the user checked-in anonymously without being signed into their account, and you cannot message them**

You will be taken to this screen where you tap on the blue "Send Message" tab:

To view your messages, tap on "Messages" on the main menu of the app.

To get notifications when you receive a message from another driver, go to "Settings" on the main menu, tap "Notifications", and switch "Messages" toggle ON: