You need to create an organisation-wide group from your Google Apps control panel:
Here's how to create an organization-wide group:
- Create a new group. Learn more
- On the "Create a new group" page, click Add all users within [organization name] to this group.
The group's member list now includes a special member called All users
within organization name.
You can edit the groups access settings or delete it, just like any
other group. However, you can't edit the members within the All users
Tip: To prevent spam being sent to everyone in your domain, allow only
group owners and managers to send messages to a group that has the All
Quoted from Apps Administration Help, here.