As you have updated it reference field 

We need to update the script to 

var group = current.variables.override_settings; //this is the variable name
var getGroupValue = new GlideRecord('u_override_settings'); //this is the table name
getGroupValue.addQuery('sys_id', 'IN',group); //this is the column on the table that is the group setting selected
getGroupValue.query();
while(getGroupValue.next()){
 answer.push(getGroupValue.u_functional_approver_group.toString()); // this is the asscociated approver group
}