Currently, you can't include a Report Variable in a Group Section with an aggregation operation performed on it (Sum, Count, etc.).
Here is a screenshot that shows the error that you get when you create a simple variable in the Detail Section of the report and then try to perform a Sum operation on it in a Group Section of the report:
Variables can often contain complicated business logic that would be cumbersome to have to reproduce in every place that you want to perform aggregation. It would be MUCH SIMPLER to simply be able to aggregate on the variables themselves.