Yes, Virginia, there is a Groove SP1!
If you have installed Microsoft Office Enterprise 2007 or Microsoft Office Groove 2007 the SP1 build number for Groove will show as 12.0.6215.
You must refer to Help-->About Microsoft Office Groove to find the Office Build Number. 12.0.6215 = SP1 for Office Client, and includes all SP1 fixes for Groove 2007.
The Groove 2007 client and Groove Server 2007 fixes are documented here: http://download.microsoft.com/download/B/C/F/BCFBEC81-ED9E-40A3-BBF0-1410B67A365A/2007%20Office%20Servers%20Service%20Pack%201%20Changes_all.xls .