I've started using some dried elderberries in the primary for more highly concentrated kits (like a 5.5L Fontana Malbec and a 10L WE Trinity Red) - basically 1/2 cup dried elderberries along with 3/4 cup zante currants. It's too early early to give any definitive comments, as I just started the Trinity Red. But I did just rack the Fontana Malbec (made to 5 gallons) from secondary for a relatively brief (2-month) bulk aging period in a carboy before clearing/bottling, and if the taste I took is any indication, the results seem promising.
By the way, elderberries themselves are not very sweet at all. But they can add an interesting flavor dimension. You'd need something much more concentrated than adding a cup or two to a batch of wine in order to get any discernible health benefits from the elderberries - do some searches for "making elderberry extract" if you want to leverage elderberries for their immunity-boosting properties.
Two years ago I made 4 gallons of Marquette and one gallon of Elderberry. I used five pounds of fresh berries to a gallon, and honey to 1.095. Some have said it's the best wine they ever had. I fermented them together.
The 1.095 is the starting specific gravity of the must. The high specific gravity is due to the sugar (or honey) content. Measuring that at the beginning and end tells you how much alcohol was produced. I use elderberries and muscadines together in primary fermentation every year. The elderberries are less acidic than you want your must to be and the muscadines are more acidic. Plus the flavor of the end product is good. I keep the elderberries in a mesh bag in the must because the little seeds will get in my wine thief and clog the valve and cause the hydrometer to stick. The elderberries are faintly sweet and mellow flavored.
Cuz What garymc said plus: I had enough pounds of grapes to end up with 4 gallons of wine, I wanted 5. The original must was a little high on acidity.. So I added 1 gallon of water, and 5 pounds of fresh elderberries. The elderberries are high in tannins and low in acid, my overall acid came down to like 3.5-3.6 and I ended up with a wine that people like with strong tannic structure. Elderberries are the one fruit that will give a wine similar too, equal too or better than grape wine. Elderberries are much overlooked and I would say it is a wine that merits an addition in every home cellar. My opinion is that while dried fruit is an excellent additive in and of itself, it adds an oxidized taste. But dried elderberries can make an excellent beverage