How do I force text messages to send and receive via iMessage?
Try disabling iMessage in Settings &rarr; Messages &rarr; iMessage. Turn it off and then turn it back on. After doing this, close all apps and restart your phone by holding down the Lock button and the Home button for ten seconds. If this does not resolve the issue, it may related to the other users' devices. 

You can also try restoring your iPhone though iTunes and setting up as a new device if you don't believe it's related to the other users.