creaet a start date and end date param.
ifyou want to hard code these to the last month youc an chaneg it, either wise you will want to use 2 paramters fields as date types
create a formula field as
//groupflag
if {PURCHASE_ORDER.DESIRED_RECV_DATE} in {?startdate} to {?enddate} then 1
sum this at the group PO level
any group hat has a value >0 has at least one row of data that falls in your run time parameter dates so you can use this to do a group select
open the select expert and expans it to show formula if it is not already
select the 'group selection' radio button
add your group selection here
sum(groupflag,po)>0
This measn you are always pulling in all of your PO rows first, grouping and then doing a group select (not efficient). If you have a way to limit the data on PO that you know will not meet your group selection use it too as it will speed thing up.
Also note that all groups still appear in your group tree even if they do not appear in the report (omitted via the group select). The group tree is created before the group select is applied.
summarization formulas are applied to all of the data (hence the ability to use it in the group select statement) but Running Totals and variable formulas should only be applied to data that 'is left' after the group select is applied.