Let's check it out by looking at some examples before coming to a conclusion.
Some rites are very exclusive. They even are worded to only allow some groups to teach or learn and act this rite. One such example is the Silver Fang rite Enter the Dark UmbraTribebook 10 Silver Fangs p52.
Only a Death Breath-spirit may teach this rite and only a Silent Strider and member of the Ivory Priesthood may practice it. [...]
It is in the Silver Fang book, but even among Silver Fangs, only one House practices it, while this rite is also known and practiced by the whole tribe of Silent Striders. In this case, no other Garou that does not qualify these, not even on an individual base, should be able to learn it.
Very tribe specific
Let's take a look at a random example of a rite that is quite tribe specific but not worded as exclusive. One example is the Glass Walkers rite All Machines DayTribebook 6 Glass Walkers p36/53.
All Machines Day celebrates our dependence on the
Machine and our reverence toward technological spirits.
The festival begins with Vulcans Day on the preceding
Friday and ends with Sister Science Day on the following
Saturday, although the moot itself takes place on the Ides
of March, the 15th, each year. On this day, we clean and repair our machinery from the early morning until the
evening. This includes the cleansing of both home and
office with rites and rituals. As the sun sets, we power up
our machinery and invite the spirits to frolic with us
through a festival of light and sound. The festivities go
into the Penumbra where we commune with the technological
spirits who aid us in our daily lives. When the
celebration finally dies down, a peaceful calm settles
within both the structure that houses the main festivities
and the Penumbra that surrounds it.
All hail the Machine!
This rite doesn't really make sense outside of the Glass Walkers, as they are the only ones that share this connection to machinery. It is rooted in their tribal culture and has no reason to exist in this or any other way in any other tribe. These I would not allow in other tribes in general, but a non-Glass Walker member of a pack dominated by Glass Walkers might undergo cultural influence by them enough to share this sentiment and then, as an individual, join this rite and possibly learn it.
Known by all, but not practiced
While much less tribe specific or even a shared knwledge, some rites are just not performed by other groups. One such is the Silver Fang Rite of the Honorable OathTribebook 10 Silver Fangs p51.
A somewhat less potent version of the Gift Honor Pact, this rite allows the Garou to swear a binding oath of allegiance to another being. [...] Most Garou know about this rite and are far more apt to trust a Silver Fang who takes it. Pressing a Fang to do so is very bad form.
This is a rare example of a rite, that most Garou know about it, but the wording implies that it is most commonly to almost exclusively performed by Silver Fangs (and their packs). I would, however, allow other, similarly honor-bound or oathbound groups to have this, or at least a very similar, rite. One group that, on my table, would clearly qualify would be the Hand of Tyr ("I swear to hunt down this criminal and bring him to Tyr's Justice!").
Universal (after Reskinning)
Other rites can be easily reskinned to match other tribes values. One such example is the Fianna Rite of InspirationTribebook 4 Fianna p49.
Rite or Inspination or “Awen’s Blessin(Level 1 Mythical)
This rite is used by a Fianna who is seeking inspiration for
a poem, song, story or any artistic endeavor. The Garou must
eat boiled pork while lying down in a dark room with his eyes
covered and a stone on his stomach. If successful, he will be
granted an idea or some solution to a problem he was facing
in his art.
In the way this is worded, it is clearly a Fianna one, but other tribes might have a similar rite. For example a very easy reskin would be to change the artistic target to one of craftsmanship or skaldic poetry, switch the pork for mead and have the Garou demand to drink themselves into a stupor and voila, we got a Get of Fenris rite (for Theurges and craftsmen). Or choose a sparse meal and seclusion into a cell to meditate and we got a gift for Stargazers. After a simple enough reskin, such a Rite can easily exist among most tribes in some fashion or another.
It is a big thing of GM Fiat, in which category a specific rite should be placed.
Keep in mind the characteristic of the tribe the Garou belongs to that wants to learn the rite: a Red Talon usually has little to no need for a rite that deals with humans or machinery, as that is something the tribe in general shuns, but rites that deal with wilderness might be all the more common among them, even if coming from other tribes originally.
Another thing to consider is if you happen to play in an area that has a different organization: A Glasswalker in Shanghai might want to learn a courtly Beast Court rite or two because he is in contact with them all the time. For example, he might want to learn the Kitsune rite "Proprieties of Conduct"W20 Changing Breeds p135. In this case, his trouble would be to find a willing teacher. Provided he finds one, nothing in the wording precludes him - in fact, if he finds a teacher offering to perform this rite might increase his standing as a gaijin...
Other ways to acquire a rite could be joining a group. For example a Strider in Africa who meets the Ahadi: once he joins, the Ahadi rites open to him. Before, it is a nogo as all Ahadi rites are Ahadi Exclusive, but once one oins a member one has access to all the rites as they are shared among all the Changing Breeds that join the pact. (same goes for most Beast Court rites btw.)
I strongly suggest keeping in mind Werewolf Players Guid (2nd) page 189 under the headline "Trade Secrets", where one can learn about cross-learning gifts and rites, and what should be used as guidelines when doing this. On that page we find this excerpt:
indeed theoretically possible for a Garou to learn an exclusively
Rokea Gift or rite, for example — but only under a few
- cannot rely on the unique
capabilities of its parent Changing Breed[...]
- any rites
that strike the Storyteller as completely exclusive to one
type of shapeshifter are off-limits.
- It's up to the
Storyteller to determine which tricks can be shared and
which can't — and her word goes.