It can be acquired from the Beast Stone after completing the quest "Cleansing the Stones". Like all powers from the All-Maker stones, it can only be used once. To use it again, it must be reacquired from the stone.
"Summons a werebear for 60 seconds wherever the caster is pointing. Single use, then must be reacquired at the Beast Stone."
- The Conjure Werebear power might only work on Solstheim and not the mainland of Skyrim itself.