"Seamless" multipleyer was one of the talking points of Watch Dogs 2, but when it finally launched, the feature was disabled because of crashing and lag, The dev team is still working on the feature and hopes to have a solution soon.
"The Watch Dogs 2 development team has prioritized finding a solution for activating the seamless multiplayer feature in the game," Ubisoft said in a statement. "They are currently conducting extensive internal testing of a solution to both ensure it causes no regression to the core single player experience as well as smoothly integrates the seamless multiplayer. If the solution works as intended, we hope to permanently flip the switch to turn on seamless multiplayer as soon as this weekend."
When the feature is eventually added, it will be on for all players by default. If you prefer to play by your lonesome, you will need to turn the mode off.
The single-player experience is unaffected, and players can still join into co-op if they want to play with friends. We found the game entertaining and worth your time iof hacking through San Francisco interests you.