An additional resource group that contains no resources is created after you install a second failover clustered instance of SQL Server 2005 on a Windows Server 2008-based server
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Product: Microsoft SQL Server 2005 Enterprise Edition & SQL Server 2005 Standard Edition
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An additional resource group that contains no resources is created after you install a second failover clustered instance of SQL Server 2005 on a Windows Server 2008-based server
http://support.microsoft.com/kb/948208/en-US