I'm sure this has been mentioned somewhere before, but I could not find any info on it. My friend and I were trying to share the aircraft on the CS767 and every time he tries to connect, it says the aircraft is no longer shared and then he gets an error about the gauges not syncing. We both have the same FSX version and CS767 version.

I heard it's some issue about custom gauges so if that's the case then does that mean only default planes can be shared? There has to be a way to make custom planes work unless it's something that the plane developer has to incorporate himself. I was hoping that someone has written a program to allow sharing of any aircraft. It was a big disappointment when we could not join the same aircraft. If this isn't possible at this point, then I suppose I could put this in the "It'd be nice to have" section.

You're right, this has been mentioned before.
Captain Sim doesn't support multiplayer operations at this moment, so I'm afraid it just won't work.
The ability to share the cockpit needs to be built in by the developer (or rather, not built out) it is indeed, as you say, the custom gauges and programming that make in incompatible.
I think there are already topics on this in just about any CS it'd be nice topic, for every aircraft.

maybe, just maybe, somewhere in the future, who knows...