Try this script. It will add the roles in the related list as well.

//Find a user eg.abc
var gUser = new GlideRecord('sys_user');
gUser.addEncodedQuery('user_name=abc');
gUser.query();
if(gUser.next()){

//Declare roles in array
var arr = ["role1","role2","role3"];

//Assign the roles 
var grd = new GlideRecord('sys_user_has_role');
for(var i =0; i<arr.length; i++){
   grd.initialize();
   grd.user = gUser.sys_id;
   grd.setDisplayValue('role',arr[i]); 
   grd.insert();
 }

}