Right click your table and go to Properties. Not the little window on the side, but the actual Table properties where there's a tab called Grouping.
Click Add, name your new group and hit the down arrow where it gives you a list of columns. Click on Expression instead of a column list. Then put an IIF() function into the expression. IIF(Param = A, Fields!Column1.Value, Fields!Column2.Value) for example would check the Parameter. If the parameter equals 'A', then it would group on Column1, else it would group on Column2.
I haven't actually done this myself, but I see no reason why it shouldn't work.
Let us know if it does or does not.
Catadmin - MCDBA, MCSA
"If a person is Microsoft Certified, does that mean that Microsoft pays the bills for the funny white jackets that tie in the back???