Hey, man! I'm in the area (sort of) and I know how to use elderberries!
Seriously, they are an intensely flavored fruit, and unless you like the really dark, really thick fruit-bomb style melomels that I make, a little will go a long way. By way of reference, for most dark berry mels (blackberry, blueberry, etc.) I'd use 20 to 25 lbs of fruit in a 6 gallon primary. With elderberries, I find that 12 to 15 lbs is enough. And I suspect that most folks would be totally satisfied with 9-12 lbs - YMMV.A couple of things to keep in mind about the fruit. First, make sure that they were picked absolutely ripe. They tend to darken before they are fully ripe, so a lot of folks pick them too early. They don't develop maximum flavor until the dark fruit literally "droops" from the stems that hold them in a cluster. That's when they are fully ready. It is also when they contain the minimum amount of the stuff that creates "green goo" on your equipment.Also, take care to separate the fruit from their stems, and discard all the stems if you can. They are both excessively tannic, and they are a much more copious source of the plant protein that forms that dreaded green goo. One fact of life when working with this fruit, is that the sticky stuff will get all over your primary fermenter, any stirring utensils that you use, and your racking cane. Unfortunately some of the green goo will come from the fruit even if no stems are present, unless you raise them up to pasteurization temperatures (155 to 160F for 10 minutes or so). I don't like to heat my elders, so I live with the sticky green stuff and I spend the extra time needed to clean all my equipment thoroughly in three stages, first with vegetable oil (the only stuff that the goo is completely soluble in), then with a quick wipe using a Goo-Gone saturated rag, and finally with a detergent mixed with Oxiclean. Only after all the sticky, messy gunk is all gone can you properly sanitize your equipment for re-use. It is a lot of extra work, but the results are worth it. I have an elderberry - black currant batch that has been bulk aging for about a year now that is absolutely the most incredible concoction that I've ever made with fruit. I really like working with them, despite all the problems associated with it.