Set value from Javascript

Hi All,
I have a method in Javacsript which gives me ID:
function playMovie() {                             if (ID == 0) {                 alert("hello");                ID = PageMethods.addE(username);             }
}

I what to use it in code behind for specific user. How can I send this value to code behind?
Can I use hiddenField? Would it matters if other person will invoke function playMovie() ?