Dark purple elderberries (Sambucus nigra, S. canadensis) are a fantastic ingredient for baked goods, jelly, pancakes, and homemade wine. You can make a terrific fruit-flavored vinegar with them. They are also an anti-viral medicinal herb, taken either as a syrup or a tincture.
If you've got an abundance of elderberries, freezing is a great way to preserve them for future use (or stockpile them until you have enough for a recipe).
I also recommend freezing elderberries before trying to de-stem them even if you plan to use them right away.
The flowers and fruits are the only edible parts of the elderberry shrub: leaves, stems, etc. are toxic. So you need to remove the delicious elderberries from their stems. It's tricky to get the juicy berries off of the stems without squishing them, but once they are frozen you can roll them off of the stems quite easily.
Snip the whole elderberry clusters off of the shrubs, still attached to their main stalks. Rinse them under water to remove any insects or debris. Spread them on a dishtowel to dry off for a few minutes.
If you won't get around to de-stemming the berries for more than a few hours, simply put the whole elderberry clusters into freezer bags or containers. Pack them in loosely so as not to crush any of the fruit. Seal tightly and freeze.
If you will de-stem the elderberries the same day, spread them out on a cookie sheet and freeze, uncovered for 1 to 2 hours.
Either way, the next step is to de-stem the frozen elderberries. Once they are frozen solid they are quite easy to remove from the stems by hand. Discard the inedible stems.
If you are working with a very large haul of elderberries, you may find that they start to thaw out quicker than you can de-stem them.
To avoid this, simply work in smaller batches, only taking a few of the still-on-the-stem elderberry clusters out of the freezer at a time.
Also helpful when you work with elderberries in quantity is returning the just-stemmed, still-frozen elderberries to the freezer quickly. If they have already thawed when you refreeze them, the result will be a big solid brick of fruit. If they are still frozen, the individual elderberries will stay loose in their containers or bags. That makes it easier to take out just what you need when you are ready to use them in a recipe.
Transfer the de-stemmed elderberries to freezer bags or containers, seal, and return them to the freezer. I prefer to use BPA-free freezer containers rather than plastic ones.