Good luck to MS holding that up in court. Prior art ffs.
You can patent anything nowadays, especially in Europe, but simply having the patent means jack shit unless a court backs the patent during an infringement lawsuit -- at least in most jurisdictions and to my knowledge.
One example is Toshiba's carte blanche patent on all forms of voice-control (which they don't even have of course, they just patented the idea). Another is Pfizer's patent on human genes. They have reserved the right to charge royalties from every living person. Good luck trying that for real though.
And really, who are they going to sue? The developers? Not a good idea. Sony or Nintendo? Well ... they have a few patents of their own that they haven't pressed Microsoft for.
I think this should be seen as MS loading up on guns for future patent wars.
By the way, the patent actually covers any "squad-based shooter" that "allows players to dynamically join and leave the game, while that game is in progress, without the players having to save and restart the game." Online, offline, co-op, head-to-head, versus .... you name it.
Like I said though, PRIOR ART FFS