Are you using the Table control or the List control?
The Table control allows you to use functions in the fields like SUM & COUNT in your groupings. You could then set the hidden property of the selected field on the table control to designate whether or not you want it displayed (your hiddden fields).
The List control is a little more tedious as you would have to create groups by embedding another list control (or other report control) within your initial list to make a grouping.
Open the expression builder on the field you want to use. Then use the Count() function and then select from the fields list the field you want counted. Be careful though, I would recommend opening the tree branch designated Datasets, then select your dataset that has the ID you want counted. You will notice that VS lists the fields with the function most commonly used based on the data type of the fields listed. I have used a FIRST(<<fieldname>>
function within the count function with success.
Hope this helps.