Joining a Public Group
In Exchange 2010, use the Global Address List (GAL) to search for exisitng distribution groups. This allows you to locate groups that might be of interest to you or assist you in performing your job duties; once located you can identify the group owner and request to join the group.* Access this function through the Options menu in Outlook Web App (OWA).
*The decision to allow requests to join a specific group is determined by the group owner.