You could either go to report->edit selection formula->GROUP and enter:
{table.effectivedate} = maximum({table.effectivedate},{table.group}) //assuming you want the most recent per group
Or, you may be able to create a SQL expression {%maxdate}:
(select max(AKA.`effectivedate`) from Table AKA where
AKA.`groupfield` = Table.`groupfield`)
You would substitute your table name for "Table" and your exact field names for "effectivedate" and "groupfield". Leave "AKA" as is, since it is an alias field name. Then go to edit selection formula ->RECORD and enter:
{table.effectivedate} = {%maxdate}
With the latter method you will be able to use inserted summaries for calculations. The first solution might require you to use running totals, depending upon the calculation.
-LB