Or you can go to report->selection formula->GROUP and enter:
{table.date} = maximum({table.date},{table.group})
This assumes that you want the most recent date per some group.
Or you could create a command:
select table.`groupfield`, max(table.`date`) as maxdate
from `table`table
group by table.`groupfield`
Link that to the main table on the group field, and then in the record selection formula use:
{table.date} = {command.maxdate}
In the first scenario, any inserted summaries would include non-group selected summaries. Using the command allows you to return only the records that are the most recent, and you can therefore insert summaries safely.
-LB