Recently, some of my co-workers and myself took another coworker out for a surprise drinks and dinner. Apparently, the someone in the office heard of this and informed the boss that we had not invited the rest of the department to join. After my boss was informed of this, she contacted us after work hours to ask why the rest of the department was not invited and how rude and excluding out behavior was! Does she have the right to intervene? How can I professionally address this with her since she obviously has some work/personal life lines blurred. What suggestions do you have so that we can maintain personal (and private) relationships outside of work?

Was this a work related celebration or personal ?

It appears the "boss" did this talk after hours, so it was not done officially and was only done to explain how it was in poor taste not to invite everone.

And why was everyone not invited ?

What type of celebration?

Was it related to work at all ?

Personal. A coworker, who is shy, is getting married.

To explain further, out department is separated into 3 subspecialties and the secretaries. One of the groups went out as we work very closely with each other and have minimal contact with the other 2 groups in our department. No malice was meant; our coworker is getting married and she would only have wanted those she had a personal relationship outside of work to attend.

I agree with you that management overstepped boundaries. Of course saying so is another matter... you explained why you did it very well, and if that wasn't accepted, I don't know what else to do or say. Hope it blows over, and be aware for the future. Perhaps apologize for not making such arrangements much more secretly. In the workplace, we often have to cave in.

I once started a part time temp job and was talking to a coworker after hours. The supervisor was listening in from 10' away, and when I said I didn't know my schedule, she harshly interrupted and told me that wasn't true at all, that it was posted on the wall. Truth was the schedule had me down for full time so it was wrong. I never got over the way she yelled. I did quit the job a few weeks later but for other reasons. Most of the time, your choices are limited to quitting.

The supervisor would appear to be dead wrong, but was most likely responding to complaints from some of her other employees.
But as joy indicated, this could affect your working conditions and career, based on how you handle it.
This might be a good time to think about moving on if there are opportunities elsewhere.

BUT, you just say "some of my co-workers and myself took another coworker out...". I was thinking 3 or 4 people total.
If it was significantly more people, I might have to rethink my response.
If the supervisor determined that this could affect the morale of her employees and the relationships between employees and between different departments, maybe she wasn't as far off base as it appears at first glance.

Here's one other theory: Whoever set up this surprise event, must have had to assume that they knew which other employees the "surprisee" knew well in the company. If it was a surprise, the employee setting it up couldn't ask the "surprisee" who to invite. Maybe there were some other employees who felt that they had a close relationship with the "surprisee" and were offended to know that not only were they not invited but that what they thought was a good relationship was totally ignored. If this scenario had happened to me (a male) I would have not complained to the boss, but the boss may have overheard some grumblings about it.

Tell the boss okay, sorry no malice and nothing personal and let it drop. Be aware that somebody is watching and complaining. Who it is? No telling, but best to let it blow over. Maybe your boss was just making you aware of the office politics without snitching and pointing fingers.