Context is everything - so it will always depend on what they are responding too.
In general it's proper decorum to consider it private correspondence.
Re-adding the rest of the group will depend on the content of the response.
If you feel that it was intended as private then you should absolutely not re-add everyone.
If it was a response to something simple that the greater group would benefit from then ask if they mind you sharing the content.
In some cases people reply individually because they do not want to bother the list with hundreds of emails or simply to save you from a group embarrassment.
workplace.stackexchange.com/users/72024/Burrito Friday
In general, when I get something privately(or send something privately) in the middle of a long group chain of emails, it is because the person on the other end wants to bring up a point without getting others, perhaps very senior others or clients) involved.
If my response to that email needs to be sent to the group, then usually I discuss it with the person who sent me the private email and then add information to the original chain of emails rather than re-adding people to a private email.
This keeps what was supposed to be private, private and puts the information that needs to go public, public.
This prevents embarrassment and allows people to feel as if their private communications are not going public which tends to result in people being more willing to give you private information that perhaps they would not want senior managers to see.
So for instance:
There is a long email chain on a data import problem that includes the client.
An Account Manager privately emails me to tell me that the client is upset that something has not been resolved and can I prioritize this.
I call her on the phone and tell her the problem is at the client end and together we decide how to respond to the original email.
Then one of us sends the response we agreed on to the entire group as a response to the original email chain.
This also has the advantage of keeping all info that should be in the entire chain in it and and adds it to the latest response (because we may have gotten more in the meantime) so that all information is in one chain.
Be especially careful about re-adding people if the private message was blunt or disparaging of someone in the chain or their management.
Suppose the private email said something like, "
I know Joe is being a jerk here, but he does need to get an answer to the CEO today.
I'm swamped, can you do me a big favor and get him off my back?"
clearly adding other people back into an email like that is going to upset Joe, upset other managers and possibly make sure the person who wrote it doesn't ever help you out in the future.
It will also be a clue to everyone in the chain that you are not to be trusted.
If the email seemed to be an obvious error (i.e., there is no reason why it should not have gone to the whole chain), then I usually email the person back to ask them to include everyone.
That saves face for them which is almost always a good thing to do.
workplace.stackexchange.com/users/93/HLGEM
If someone sent you a private message then that person made a concious decision to contact you – and only you – privately.
They may have done so simply because they thought the rest of the group wasn't interested in this specific message, or they may have done so because they didn't want to send that message to the rest of the group for any number of reasons.
Telling the difference between the two is not always easy, and the safe path is always to ask : "is it okay if I forward out correspondence to the rest of the group?"
workplace.stackexchange.com/users/8388/Martin Tournoij
You can send your message to whomever you want.
Where you'll get into trouble is forwarding somebody else's private message without consent.
Most email programs these days include the replied-to message by default, so reply + CCing others causes them to see the private message.
Watch out for that.
One way around this is to ask for permission to share, as others have suggested.
Another way -- IMO better when the thread is already long -- is to address the point without forwarding.
Something like: I was asked how we'll guarantee data integrity when doing large asynchronous writes.
We plan to do this by...
Or: Several people have asked me whether the new match for our retirement plan is retroactive to the beginning of the year.
[answer] If somebody made an astute point and you want to give credit, get permission and then say something like:
Alice pointed out that the export operation could run for a long time and holding the global lock until the end could prevent backups from running.
We'll address this by...
workplace.stackexchange.com/users/325/Monica Cellio
I make too often the mistake of answering privately while intending to answer to the whole group.
In such occasions usually the person answered just answers back to me pointing that I seem to have clicked the wrong button.
Then I tank him and resend the original message to the group.
I think this is the obvious and most safe and respectful course of actions.
However, if it were clearly obvious that the message was intended to the group and waiting to the sender to fix the mistake could harm communication with an undesirable delay - for example, if there was a deadline on the debated issue - it would be fine to CC everybody again in the answer.
workplace.stackexchange.com/users/54672/Pere
If someone sends an email to a group of people and a member of that group intentionally replies privately to the original sender (and not a mistake of forgetting to reply-all), then is it considered a private correspondence?  Is it acceptable for the original sender to re-add some or all members of the group to the reply of the private message?
Is it unprofessional to add people back to the CCList when replying privately to the original sender?
workplace.stackexchange.com
<professionalism><email>
workplace.stackexchange.com/questions/93656
workplace.stackexchange.com/users/48161/Sean Glover
It depends on the context of the message. If you think it was intended as private then you shouldn't add everyone back in. However, if it was clearly intended for the group then go ahead and reply to all. The answer to this question will largely depend on context, through some people will say that you are free to make your own decision as to whether people are added to the exchange or not. If you feel that the message was indeed intended for the whole group, or would be more beneficial for the whole group to see, then you may wish to re-add them to the e-mail. If, on the other hand, it is obviously meant as a private message to you, you may want to err on the side of caution and not re-add anyone to the exchange.
The answer to this question will depend on the context of the message sent and the reason it was sent. If there is reason to believe that the person wanted to include the whole group, or that it would do more harm than good for the group not to have seen the message, then you can consider re-adding them. Some people believe that you have complete freedom to add whoever you want to an e-mail exchange. If you feel that this was a private message, then you might want to err on the side of caution and not include anyone else on your reply.