In the Online Contracts App accessed from the in-game smart phone, there are two modes ("Online Tailing" and "Online Hacking"). I assume these are the modes where other players can jump into my live game, or I can jump into theirs.
These modes both say "Complete Online Contracts Introduction to unlock". Where is this introduction?
Note I have just recently unlocked the second area (The Loop), and am at 3/9 in Act 1, for some idea of my progression through the game.
Best Answer
The answer showed up just after I posted this question.
As I reached the location for the next mission ("Backseat Driver") and attempted to start the mission, I was informed that another fixer was attempting to hack me.
The game then required me to navigate to an area indicated on the map by a purple circle. When I reached that location (for me, this was the parking garage where I'd just completed a side-mission) I had to scan people until I found the rival fixer (it appears that this is always a bot called 2XTheTap) and chase him down and shoot him.
After this, I was able to access both the Online Hacking and Online Tailing modes.
Some research shows this is the standard way to unlock this feature.