Stack Exchange chat crash course 101 for dummies
One network, three chat "zones"
First of all, one needs to know that Stack Exchange has three chat "zones", which are:
- Stack Overflow
- Meta Stack Exchange
- All the rest
Each of those zones has its own rooms.
Viewing existing chat rooms
To enter an existing room, just browse the full list of rooms in any of the zones and click on the desired room. If you're logged into the chat (you can see that in the top bar) then you'll automatically join the room when you enter it, and all users in the room will see you join. Otherwise, you'll just see the activity in the room but nobody will know you're there.
Taking part in chat aka writing messages in existing room
Chatting in a room requires you to login with an account having at least 20 reputation points. For Stack Overflow chat rooms you need 20 points on Stack Overflow itself, for MSE you need 20 points here on this very site, and for the rest you need 20 points on at least one site in the network.
Creating new chat room
In order to create a new room, you must have the Create Chat Rooms privilege, which requires 100 reputation points. If you're logged into the lobby of a zone and you have enough reputation, you'll see a "create a new room" button at the bottom:
Clicking on it will allow you to create a brand new room. There are theoretically no restrictions, but keep in mind you can't delete the room once it's created, so think twice before doing so.
Inviting a user to chat
The most straightforward way is to create a chat room, and then post a comment on one of the user's posts, or as reply to one of his/her comments linking to the room - use the URL you see when you view the room itself. Once the user clicks on the link, he'll enter the room.
However, there is also a more elegant way when the user you want to invite already has a chat profile. In this case you don't need to create a room beforehand. Go to the proper zone, e.g. Stack Overflow, then click on the "users" tab:
Here you can search for the user you want to invite. If you find him/her click on the name to reach his/her chat profile. Once there you can click on "start a new room with this user":
After confirming, a room will be automatically created and the other user will get a notification in his/her inbox with an invitation to the new room. All you have to do is wait for him/her in the room. (All the rooms named "Room for X and Y" were created this way)
That's it! Enjoy chatting! :)