NEED HELP sorta

Yes. The problem he's having is that his predicted movement is off when the player looks in a different direction too quickly and the syncronized player ends up not being in the spot it should be. There isn't quite a perfect fix as long as there's lag, but updating the players coordinates when they're wrong is a way to make up for the error. It's sort of how the system can admit it's wrong and make up for the mistake before it causes a problem, or problems.

It's a good idea to keep the predicted movement, but also send the players coordinates, that way when the game slows down your character can jump to where he should be if his predicted movement is a bit off. you'll see it happen in all major online games. players will jump after a spell of lag. It doesn't look pretty, but it does work fairly effectively. Also try improving other things that cause a lot of lag. It might be that the lag is unnecessary after some quick fixes.

Well, I go about things like this with scripts, but you could do it within an object. The code is the same either way. However if you need drag, and drop, it's going to be harder.
If you wouldn't mind, I think it would be helpful for everyone if you'd tell us a little about the style of game, and any details you can.
It will be helpful.
Thanks.

lol, yeah I'd agree for sure.
You probably will have to do something like:

global.controlid=id

in the controller's create event. Then you can just check like this:

  with(all)
  {
   if id!=global.controlid
   {
   instance_destroy();
  }
  room=mainmenuRoom;
  break;
}

There are a lot of different ways to do that though. This should work though.

Edit: Ah, I type slow. That's a good solution that c_raethke has given.

Well, there are a lot of ways to go about it. All of which are going to take time, and tricks... lol.
The way I've done external rooms is with a level editor, that saves the room files, and the game loading the files in with as little calculations going as I could manage.
For your case you may want a level editor that's a little different than what mine used. You may want to make a system that will save the rooms you already have to file.
What it would need to do is save essential general data (room name, background, view sizes perhaps, etc.) and then save the stationary objects' locations and names. Then you'll need to run through the rooms you want to save running the script, and it will save those object names, and locations, and all of those necessary details to a text file(I would encrypt it, but that's up to you). Then what you'd need to do is (basically) clean out all of those rooms, and make a room loading script that will load all of those objects, and room details for wherever you are.
It's easier explained that way than to show you code right off, but I can put some examples on here if you like.
Systems like this fascinate me, and they're fun to make - which is probably why I mostly don't finish projects, I get too caught up in the details.
Anyway, I hope that helps you out - however generalized it may be.

Edit: to be more helpful... run through the objects you want to save, and not the others, like the character, and any moving objects. I really wouldn't write the file by hand when a script may be sufficient. I'm not saying it's a bad idea at all, but it may be easier to write up a script - especially if you're doing a lot of rooms, or you have a lot already.