How to apply filter at runtime in SSRS 2008 R2
Probably the easiest way would be to use a drill through report. Add an action to the row group that links to a new report. Duplicate the layout of the new report, but implement a filter on the dataset of interest. 

Pass the row group value ([SQLFieldName]) to a parameter of the report. If you use a multivalue variable with an IN statement in the SQL, several filters can be used at once.