I followed the steps described on http://blogs.msdn.com/robertbruckner/archive/2008/07/20/Using-group-variables-in-reporting-services-2008-for-custom-aggregation.aspx
and kept getting this error
error: all tablixmember elements in a tablixcolumnhierarchy must have the repeatonnewpage property set to false
the reason i was getting this error is because I was setting the repeatonnewpage property to true on the wrong static.
so here are the steps again.
1.) Bottom right there is a little black triangle next to Column Groups, select "Advanced Mode" from the drop down
2.) click on the "(Static)" row under the group you are using, make sure your Properties (Solution Explorer) is open.
3.) Set FixedData True & RepeatOnNewPage True
the other post has a diagram thats not working