If you're a co-op Leader or Co-leader, the game lets you send a message to all your co-op members at once. Find the Send Messagebutton in the Co-op Building interface, open the message window, write your message, and then tap the Sendbutton.
Your co-op members will see an envelope icon next to the co-op chat and can read the message in theInboxtab of the co-op window.