Displaying list of users with Cascading parameters

Hi

I have to try and get a list of users in SSRS.
I have 2 select options for them and if A selected from the first list then the second list should show all the values starting with A.
If I am selecting A and B from the first list then the second list should show all the values starting with A & Starting with B and so on

Both the user should be able to select multiple options from both the select list 

Rgds,

Shree