Use Powershell in Exchange 2010 to give a distribution group permissions to a security group's member's calendars'

The first thing you will want to do is create and active directory security group.  In my example I create one named “calendartest”.  Now I’m going to add all the users who need to have their calendar shared with a distribution group of users to this group that I’ve created. 
Now open adsiedit.msc so you can find the location of the group in active directory.  You are going to need this.  In my example it is “CN=calendartest,OU=MyUsers,DC=TestDomain,DC=local”.
Finally open powershell and simply run these two commands:
$calendartest = Get-mailbox –Filter {(MemberOfGroup -eq “CN=calendartest,OU=MyUsers,DC=TestDomain,DC=local”)}
$calendartest | ForEach-Object {Add-MailboxFolderPermission $_”:Calendar” –User –AccessRights Reviewer}