I want to invite a certain user to a chat to avoid writing more and more comments on a post, how do I do it?
If the user has a chat account, go to their profile and click on the "start a new room with this user" button there.
If they don't have a chat user, you can't invite them to chat. Moderators have a way to do that, but that is not meant for this kind of situation.
I believe you can only do this via a comment that links to the chat. I think it's a shame that we have to use a comment to do this, but at least it's a comment that doesn't invite further discussion.
If they've already been a participant in a particular chat, then they will be notified by @ messages in that chat.
You can also invite a user to a room via their chat.stackexchange.com profile page: Inviting a user to chat from the profile page?