How to add a custom filter while listing users

I am working on a custom module which requires to add a new custom filter while listing users. The default filters are "Roles" , "Permission" & "status" .
I wanted to add a new filter , which i am able to add using hook_form_alter. The problem is , i am unable to perform further actions after a new filter has been created .
I created a new drop-down filter and when i select the value from it and hit "filter" , i want the list of users to be refined . How do i do this . Please help me .