Starting with what a werewolf is (forgoing Hircine's Ring as that's atypical).. a man or mer that's forced to change into an uncontrollable beast and is driven to kill, a couple times a month (or every day before the Bloodmoon). Existing information makes me believe that a person in werewolf form doesn't remember their normal self, and a person doesn't remember what they do as a werewolf. In my mind, this puts some heavy restrictions on groups forming. Assuming groups are formed as man/mer, when they change they lose all sense of allegiance they may have had.. they could be a danger to each other (it's quite likely that werewolves can attack and kill each other), as well as the general populace (and depending on the person's demeanor, more werewolves together = more killing = bad), which would not be very healthy to survival. It could draw more attention from hunters, scare people away from town, and/or drive otherwise good people insane (effectively breaking any kind of pack). There's a question if you even could befriend someone who you know regularly kills people.
If the groups form as werewolves, then their normal-selves may not even realize it. A person might get what's going on eventually, but for 20+ days a month, they still have to have some common purpose to effectively remain a group, and some of the previous points still apply. Additionally, in any significant numbers, werewolves working together would be a serious threat, and it's my understanding that werewolves are barely known to exist, let alone be a significant problem in the empire as a whole.
Going with that, then, there's really only one way I can effectively see packs forming. If there's a common purpose in looking for a cure, or in keeping each other sane, it would be easier with someone that you can talk about it with more openly instead of having to hide it. Though they still would have trouble with the locals.. more werewolves in an area means more people being killed, etc. They could try to lock themselves away when forced to transform, but I don't know if not fulfilling bloodlust is considered fatal for non-player (ie. non-special) werewolves, or what kind of psychological effects not killing could have. Otherwise, a pack that regularly hunts together would be too much of a threat to let remain, or would quickly run out of food supply. I'm not sure what else a werewolf pack/tribe/whatever could do that wouldn't involve one of these.
That's how I see it, though I'm curious about other people's thoughts.