I fly F-16s with the Air Force. There are essentially three methods of formation flying through IFR. The first is by maintaining fingertip formation (roughly 3 foot wingtip separation). Flight lead flies off his instruments while wingmen fly visually off him since you can usually see far enough to maintain visual inside of a cloud. This is normally done as a 2-ship. There are also specific procedures for deconflicting if you lose the visual. The second is instrument trail where you take off with some briefed time delay and every flight member calls their altitude over the intraflight frequency every 5000 feet during the climb and calls every turn. This is only done on departures and flight members are not allowed to be at the same altitude until in VMC and visual with each other. The final one is most common in fighters which is called a radar trail. This is where every flight member locks the one in front of them with their radar and deconflicts based on that.