Any member of your team can be a chat user.  Because chat is licensed per user, we allow you to pick which team members have access to chat.  To activate a chat user, simply go to the User section, select a user and click edit.  Check a box to enable them as a chat user.  Once you click Save, if the user is currently logged in, they will need to log out and back in before they will see the chat button on the main left navigation.