I'm new AS User!

I have a cube that have some measure.
I have some dimension but I need a particulary measure.
Example:

Vert. Dimension Horiz. Dimension
Agent Code|Province|Cat.| Period (Year,Quarter,Month)

Measure
Amount|Quantity|

I have moreover
Number of Customer that have Invoiced
and
Number of Invoice

My problem is that my Fact Table is a Sales Invoice Line!

I set Number of Customer that have Invoice as Bill-to Customer Code with
aggregate COUNT.
I have a numer of row and not Number of Customer.
HELP!
How can I do create a calculated member that provide this information? Do
you have some example (MDX lng)?

Thank you very much.