Discussion with the whole team :
To start a discussion with your whole team (public messages), you can go to the Thread of an event: match, training, tournament, or other.
You can access it directly from the page of the event concerned, or directly from the homepage of the site, where all discussions of the team are grouped together.
Discussion with some members only :
To start a discussion with some members of your team ('private' messages), go to the 'Messages' tab > 'New Message' on the left.
You will then have to fill in the "Add recipient" field and add the person(s) to whom you wish to send the message via internal messaging (thus in private messages).