var BackfillAssignmentGroup = Class.create();
BackfillAssignmentGroup.prototype = {
 initialize: function() {
 }, BackfillAssignmentGroup:function() {
   var gp = [];
   var a = current.assigned_to;   //sys_user_grmember has the user to group relationship
   var grp = new GlideRecord('sys_user_grmember');
     if(a){
   grp.addQuery('user',a);
     }
   grp.addEncodedQuery("group.typeLIKEcff624593854010072edffb4849a2c4a"); //Set to your Group SID Type to limit amount of Groups.
   grp.query();
   while (grp.next()) {
     gp.push(grp.getValue('group'));
   }
   // return Groups where assigned to is in those groups we use IN for lists
   return 'sys_idIN' + gp.join(',');
 },
 type: 'BackfillAssignmentGroup'
};

// Below is the Advanced Reference Qualifier for the Task Table Assignment Group Refrence Field - With No Attributes Needed.
javascript:new BackfillAssignmentGroup().BackfillAssignmentGroup()