If you want to contact your colleagues, go to the menu, click on "Message" and then select "New message".
All users are as standard visible in the view so you only need to click on the ones you want to be included in the conversation. You can also search for users by clicking on "Search" under "Add new participant". The search results are filtered as soon as you enter the first letter.
After you have selected a participant, go to the bottom of the page where it says "Message to send..." to write your message. All you have to do after that, is to click on the paper plane to send your message.
You can then see the messages in the Message-view. In this view you can also answer to your colleagues' messages.