Open the Exchange Shell and type the following command:
New-Mailbox shared –shared –userprincipalname shared@jermsmit.com

If the mailbox was already a user mailbox and you are converting to shared type the following command:
Set-Mailbox -Identity shared@jermsmit.com -Type Shared

You can now verify that the box is indeed a shared mailbox by typing the following command:
Get-Mailbox -Identity shared@jermsmit.com | Format-List RecipientTypeDetails