I used elderberries in a pale ale and liked it. I used ~ 3lbs, froze them, then put them in a nylon grain bag and threw it in the kettle after flameout. After 10 minutes I lifted it and suspended it just above the wort to drain the juice out (I gave it a little squeeze with some spatulas too). The juice will stain your fingers BTW. It's on tap now and the beer is fruity/berry with a nice red color.
Elderberries contain tannins in their skins like grapes, so be careful about leaving them in contact with beer for too long and I'd really advise against pureeing them as it will speed up tannin extraction.