How to trade with someone you can't see or target?

How do people do this?

I assume there is some chat command that they use to either target or invite people they can't see, but I can't figure out how to do it...

Click on their name in the chat window, as if you were going to whisper a private message to them, then use Ctrl +C and then Ctrl+V to cut and paste their name from the chat window to the Party window.

If you are going to trade with someone chances are you have the character name in your line of site. Make sure you are in the same district as them and type in the character name into your party menu and hit enter (or the +) to add them to your party. You can then run around until the name in the party menu displays as selectable, click the name, hit the space bar, and watch your toon find them in the town.

If you are asking about something like an auction house, or /command to do this task, then sadly no, nothing like this exists in GW.