http://blog.mpecsinc.ca/2007/07/sbs-setup-public-folders-shared.html

This is our method for setting up a shared calendar, contacts list, task list, etc. at the root of the Exchange Public Folders.

1. Start your web browser on any workstation or the SBS server.
2. Open: https://mysbsserver/public
3. Logon as the domain admin
4. Right click on Public Folders and click New Folder
   Name it and select Appointment Items under the "This folder contains:"
5. Click OK
6. Log off of OWA.

We do this because adding the folder via Exchange Management does not give us the ability to share any content. It is strictly a storage folder.
We move on to:

1. Log onto the SBS server as domain admin.
2. Under the Server Management Console:
3. Advanced Management
4. Right click on First Organization (Exchange) and click on Properties
5. Put a tick beside both "Display …."
6. Click Apply and OK. (You may be warned to close the console)
7. Close the Server Management Console.
8. Open it again.
9. Again, under Advanced Management:
10. Expand the Exchange organization–>Administrative Groups–>first administrative group–>Folders
11. Right Click on the above created folder and click on Properties.
12. Click the Limits tab if you want to set limits to the folder.
13. Click the Permissions tab.
14. Click the Client permissions button.
15. Add and set permissions for those users who need access to this folder.

Now we continue on to configure Outlook.
The process is quite straight forward for adding the new folder to Outlook:

1. Open Outlook.
2. Click on the Folders View button.
3. Navigate to Public Folders–>All Public Folders.
4. Right click on the folder created above, and "Add to Favorites".
5. Name the Favorite and click ADD.
6. Click on the Calendar button in Outlook.

You will now see the newly added calendar under "Other Calendars" in Outlook.
The user will be able to click on it and work with their own calendar and the shared calendar side by side.
Permissions on the folder can be quite granular. Be very careful about giving people Delete permissions on ALL items other than their own. Only a couple of key people who understand the process should have that permission.
It is also important to let the users know that updates to the shared calendar will not necessarily be reflected at the other workstations that are connected to it right away. It sometimes takes a bit of time before updates are shown across the organization.
This process is essentially the same for all types of Exchange Public Folders shared items.
For larger organizations, adding folders at the root may make sense for organizing their content.