At the moment, you can automatically mail-enable groups in Exchange 2007 (and Exchange 2010) using a script.
1. Add the 'Run a program or PowerShell script' action to your Business Rule.
2. Select 'PowerShell script' in the Type combo box.
3. Add the following script:
$alias = "%samAccountName%"
$administrativeGroupPath = "Adaxes://CN=<Administrative Group>,CN=Administrative Groups,CN=First Organization,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=company,DC=com"
$groupPath = "Adaxes://%distinguishedName%";
$group = [Softerra.Adaxes.Adsi.AdmObjectBinder]::BindToObject($groupPath, $NULL, $FALSE, $NULL);
$group.MailEnable($alias, $NULL, $NULL, $administrativeGroupPath);
Replace <Administrative Group>, <company>, and <com> with appropriate values.