I make loads of elderberry each year and I can tell you that flavor, colour and tannin is very strong.
I use 1 part elderberry on 3 parts of water. So normally 3 kilo elderberries in 10 liter water.
Being 1# elderberries to make 3# wine.
http://wijnmaker.blogspot.com/2007/08/scroll-down-for-english-version-ik-had.html
However there is one big issue in this. The juice you received !!!!
You do not know how it was made, so the juice already could be dilluted.
If it is it would be nice how much water was added.
So before starting do get some more info on the generously received goods.
You can get a hint by reading this:
http://wijnmaker.blogspot.com/2008/08/vlierbessen-sap-maken-elderberry.html
My pure elderberry juice had an SG 0f 1.040. So yours should be in that line if it is pure.
Anyhow, get info on how the juice was made.
As they are of unknown origine, I would certainly boil them as elderberries are poisenous.
I know there is a lot of controversity about the poisenousness of elderberries but suit yourself, if you do get sick do not forget I gave a warning.
Give it some banana's for extra body and aim at a high alcohol percentage and you will have a full bodied, high flavor wine.
I like mine sweet, very sweet.
Luc