Hey Matt,
A very easy way to conditionally sum records and display on the group levels is by creating a formula with a code for e.g:
If {database_field} = "ABC" then {Measure_field}
This formula evaluates for each detail row thereby checking whether the record = "ABC". If it is then it takes that record into the consideration.
You can then simply right-click this formula field > Insert Summary > and choose the Summary Location as 'All Group Levels', or a specific group or a the Report Footer for Grand Totals.
If the records to be summed up conditionally are based on some other conditions that are based on print time functions, then you have to:
1) Create a formula that contains multiple variables that conditionally sums up the records - more like a Running Total. One variable for each group section
2) Create a reset formula that resets a particular variable on a particular group
3) Create a formula that refers to the variable and simply displays the value held by that variable meant for that specific group.
Hope this makes sense?
-Abhilash