Ok, I believe you're looking in the wrong area. MDX statements are for cubes, where the basic cube structure is known and fixed.
You're into ASSL development, which is going to require some intensive work. As well, as you alter the cube in te manner you're describing, you need to think about rebuilding your aggregations, checking / validating any scripts, etc.
I'd suggest reexamining the problem. Have you considered inverting the relationship so that you have userID, SecGroupID, and Value (Yes/No)?