Adding a CCK field with Rules I'm trying to use Rules to populate a CCK field - a user reference when they flag a node.  It basically works, but if there's already a user reference it overwrites it instead of adding another.  That's not exactly what I want.  I want the rule to add another user reference field to whichever ones are already there, then populate it with the user reference.

Do you have any idea?

Edit - What I eventually came up with, without Rules - 

function bamfmodule_flag($action, $flag, $content_id, $account) {
  if($action == 'flag') {
    if ($flag-&gt;name == 'event_bookmark') {
        $node = node_load($content_id);
        array_push($node-&gt;field_trainer, $account-&gt;uid);
        node_save($node);
    }
  }
}
Does it need to be a user reference?

Assuming that you are not using a global flag (then you wouldn't have an issue with multiple users), that information should already be stored in the flag tables. You should be able to show that information with a view (which you can attach to the node with http://drupal.org/project/views_attach). Or if that doesn't work, do a custom query in a hook_node_view() implementation in a custom module.

And if it has to be a user reference for some reason, an unlimited multi-value user reference should be easier to handle with Rules than a unlimited number of user reference fields. CCK (nor Fields, for that matter) is just not designed for that. And you can only add fields to content types, not single nodes.