Access the list of the leaders' names from the left side of the group page below the group image and send them messages by clicking the 'Send Message' links below their names. You may opt to send message to all leaders at once by selecting the 'Send Message to Leaders' link on the bottom left of the page.
Members can send messages to other members that they follow by clicking 'Members' in the main page then search for member's name. Select 'Short Chat' under member's name then fill in the required details and click 'Submit'.