I have found several questions about chatting with low-rep users, and the answers usually mention making the user visit chat so that you can invite him to a room. My question is, does just opening a tab to http://chat.stackexchange.com make someone a chat user, or does he need to visit a room, visit two rooms, ... What counts as visiting?
To create a chat user you need to log into chat. For users logged in to Stack Exchange, that is done automatically (they get logged in to chat automatically).
Visiting rooms anonymously will not do it, as anonymous users can't do anything on chat.
The chat faq has this to say:
How do I join or log in?
You must be a registered user of The Stack Exchange Network to participate in chat. If you are logged in there, you will automatically be logged in here. Your display name and gravatar are also inherited from The Stack Exchange Network.
If you want to invite someone to chat, the only single requirement is for that other user to have 20 reputation points.
Note I can't invite him to chat though, since he got less than 20 reputation.