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 "Message" text under their username:

To view your messages, tap on "Me" on the main menu of the app, then tap on "Messages":