From the looks of it, I'm not sure the idea of SP/MP is that relevant. It seems very much a 'drop in' system. Basically you play the game and if your mates show up you play with them, otherwise you play on your own. Some of the other people in the world are AI, some of them are real people. It doesn't really seem to make much of a distinction.
All the games I've seen that even vaguely carry this off before now are MMOs. Without knowing how 'persistent' the world in The Division is, it's a bit hard to tell how successful it's going to be.
