I've had occasions when someone replies to my question and I'd like to invite the person to the chat room for a few followups, instead of having a dozen comments going back and forth. How do you do this? It shows the option to move conversation to chat when the comments exceed a certain number; but I don't see the option before that. Thanks.
I think there is no such option available (I was looking for it recently with no luck)
According to Recent feature changes to Stack Exchange
2011-06-10: When adding a new comment to a post with many comments between two users in a short time, a link is provided to create a chat room to continue the conversation.
'auto chat link' is relatively new feature.
However, Can I request a chat with someone who answered my question? looks like alternative and top answer also indicates that chat link existed but was removed (if I understand it correctly)