I think that because you are running your script in the HR Scope, the gs.getUser() method is not working. I have seen similar issues when trying to run gs methods when calling them outside the scope of the include. Since we cannot use that method, I just wrote what it does in that function within the script include. Here are the new details for the script include:

var usrUtilsAjax = Class.create();

usrUtilsAjax.prototype = Object.extendsObject(AbstractAjaxProcessor, {


       /**

         * Checks to see if a user is a member of a group

         *

         * @param           {string}   sysparm_grp   The sys_id or name of the group to check

         * @param           {string}   sysparm_usr   The sys_id of the User to check

         * @return         {boolean}   Returns true if the user is a member of the group,

         *                                               otherwise returns false

         */

       chkIsMember: function() {

               var grpID = this.getParameter('sysparm_grp');

               var usrID = this.getParameter('sysparm_usr');

               var memObj = new GlideRecord('sys_user_grmember');

               var qString = 'group=' + grpID + '^user=' + usrID;

               memObj.addEncodedQuery(qString);

               memObj.query();

               return memObj.hasNext();

       },


       type: 'usrUtilsAjax'

});

Hopefully this will work.