Getting to Know You&
Should people take the time to get to know a system's headmates as individuals?
Well, for this example, lets say that you had an identical twin. You guys look almost exactly alike. You guys however have differences in personality. So, in some cases you have different responses to the same things, different opinions of people, and different tastes. Now, your twin's friend comes over to hang out with their friend, and you notify them that the person isn't avaiable. They shrug and say that they'll hang out with you. This seems nice of them. However, they can't seem to wrap their brain around the fact that you are different in meaningful ways. They shrugged and gave you a funny look when you asked them not to play your siblings favorite movie. That was innocuous enough, but when you look at the whole situation, you realize that in little ways, they expect you to be your twin. They weren't trying to hang out with you. In fact they couldn't even be bothered to get to know you. They were just expecting you to fill in for your twin! Like a stand-in, or a stunt-double!
How would you feel in this situation? Not a pleasant feeling, huh?
For all it's flaws, in some ways being plural can be like being twins with one body. Many people will expect you to be just like the "other one(s)". Carbon copies with no identity of your own. However, as similar inside and out as y'all may be (in some circumstances, this is up to and including eerily similar), there are bound to be points where you have differences.
With multiples, as with twins, these differences should be acknowledged and respected.
How should external folk deal with new people they've never met in a system?
You could try using the plural convention of saying "Welcome back" or "Welcome home" — welcome back is for folk who may not have fronted in a while, and generally a safe bet. Welcome home would be if someone had been stuck or lost, or even if they were newly created.
More important than these plural culture tidbits is treating them just like any other acquaintance like the roommate of a friend. Be cordial, have a chat, don't brush them off or make them feel like you're just waiting for your friend to come back out — treat them like pleasant company as much as possible.