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Sometimes I find questions or answers by users and I would like to ask the user a specific question that doesn't (yet) fit as a full SO question.
Adding a comment with @username doesn't necessarily seem appropriate, as it isn't a comment on the question or answer per se.
So, can I somehow open a chat or contact that user without putting in a comment?
It is allowed to start a chatroom with a user, but they are under no obligation to enter it with you and actually talk to you.
You can either request them to join by leaving a comment on their post with a link to the chatroom, or visiting their Profile on https://chat.stackoverflow.com and clicking the Start a new room with this user button. This only works if they've already been on chat before, and if you do this they will automatically be given a notification that they have been invited to join a room on chat.
It is entirely up to the user to join in either of these cases and you cannot force them.
