Adding second user to user field in plugin brings server to its knees
You've created an endless loop with calling saveEntry within the entries.saveEntry event listener. You can fix it with implementing a static variable.

craft()-&gt;on('entries.saveEntry', function(Event $event) {
    static $recursionLevel = 0;

    if ($recursionLevel == 0) {
        $recursionLevel++;

        // `saveEntry` in `addAnotherUserToNotify` triggers another
        // `entries.saveEntry` event
    }
});