Joining another player doesn't effect his lag, but you yourself may see some apparant differences. We all have lag to some extent, and when we have dealings with another player the sum total of the two players lag is the lag you deal with. So, as a joiner, you'll be dealing with lag between you and your "driver", as well as between you and anyone he comes in contact with.
The end result is that your driver won't be exactly where/when he appears to be, and neither will anyone he fights. All three of you will be seeing slightly different things. So, your "driver" will be able to kill people when it doesn't appear he has a shot, and he'll miss people you think he could hit... The same could happen with guys shooting at your driver. He may appear to be hit by people who have no possible shot.
It isn't really a huge problem. You'll still get the basic idea, and I think it's a valuable learning tool. You just need to realize that things won't be exactly as they appear on your screen. You won't be able to accurately judge how much he's leading his targets, for example.
Gauges won't all work either. I think the important ones will work, but no fuel gauge or ammo counter, and you also won't be able to see your drivers tracers.
Side switching- one hour. If you switch teams, you must remain with that team for one hour, and then your free to switch again.