How repair a joint that should have been joined by screws
Where the gap is forming you may be able to you put a couple 90&deg; angle brackets on the inside and then use lag screws or construction screws to hold it together. First screw should go on the side and then the 2nd to the back. I would first try to pull the wood back together if you can. The lag/construction screws may pull it together but I wouldn't expect it to given the size of the deck. I'd then go along and do this on a few other joists to ensure they won't pull out.