the werewolf perk tree is standalone, if you are a werewolf you have it.
there are 4 perks that go with the totems you get during the companions, though you are not required to get them.
you do not have to join the dawnguard, but you do have to get the DLC for it.
Spoiler I would suggest doing companions for "totem of the moon" though... it summons 2 werewolves who can take a dragon on their own
