You can add a Calculated field to your dataset and set its value to the difference. Then use the sum of the calculated field--> SUM(Fields!CALCFieldDifference.Value)
You can also add a group variable to the user group and set the value to the difference and, I think, you can sum the variable --> SUM(Variables!varGroupUserDifference.Value)
However, you may be preventing from aggregating a group value down, I haven't tried it.