Q. How can I add additional virtual machines (VMs) to a clustered Hyper-V application or service group?

A. You cannot have more than one VM in a service or application group using Windows Server 2008 RTM. However, Microsoft has released an update (http://support.microsoft.com/?id=951308), which modifies this behavior and allows multiple VMs to be part of a single service or application group provided they're using the same disk resource to ensure all the VMs are moved should the resource group move to another node.
The update also lets you add VM's to an existing resource group, again, assuming they share the same disk resources.