Is it rude to make 質問 about users?

Its REALLY rude,

But without them knowing または finding out

If they know about it beforehand and they say it's okay,and as long as the 質問 are not too personal it should be fine.If they don't know about the questions,then yes,it is kind of rude.

Not really, I tell あなた its perfectly fine! Except to just go to the right place to do it! c:

It depends. Though, it's better to just not make 質問 about other users.

Well I guess it just all depends

Depends on the question.

if it isn't insulting, inappropriate, または done with the wrong intentions/outcomes then i believe it is fine

I would say it is.

If あなた have a 質問 about a specific user, just ask them privately rather than putting them on blast.