There's a "central" tradition whose centrality to the group will have no bearing on anything outside of group creation. The group has to have at least one magician of the "central" tradition.

The negative modifier to creating the group for each extra tradition will be variable based on how much it diverges from the central tradition. For example, if you had a hermetic central tradition and you wanted to admit Gardnerian Wiccans, you'd have no modifiers and they could even join after group creation even if you didn't specifically include them. Same for chaos mages. Chaos magic will have few traditions outside of the aforementioned traditions with which it would have that lack of a modifier, but it will have at most a -1 modifier with any other tradition. Hermeticism will only have a -1 with Wuxing, and a -2 with hedge witchcraft.

And so on, and so forth.

So a magic group would not have to be one tradition like RAW. Different traditions have enough similarities with others to be almost interchangeable, like Chaos and Hermetic traditions? I was wondering about this when working on a chaos mage and wondering why, if the two are so similar, they couldn't use hermetic spells or be in a hermetic group.

Would this mean spell forumulas would not be exclusive to one tradition but would require a "translation" test or simply have a higher drain but be semi-interchangeable?

A small "translation" test, with the cooperation of a magician of the other tradition, just like in the chapter fiction.

Same thing for power sites. They will all have aspects based on how people draw out their magic. If they draw it from nature, then nature magic power sites will work for them no matter what their tradition, whether they are Gardnerian wicca or shamans or animists. If they draw it from the elements, then hermetics and wujen will be fine. If they draw it from the self, then Buddhists will be awesome there.

And power sites can be aspected to magical groups.

Well, this be the way I work the whole mess, which applies to aspected domains too:

Traditions & Sub-traditions – Some magical traditions are actually branches of larger or more common magical philosophies – for example, Chaos magic is an outgrowth of the Hermetic tradition. Members of a sub-tradition can work unhindered with practitioners of their tradition’s parent tradition (and with members of other sub-traditions of that tradition) in magical situations such as ritual sorcery and initiatory groups. Of the traditions noted in SR4 and Street Magic, the following are thusly related:

Parent Tradition/Related Traditions
Hermeticism: Black magic, Chaos magic*, Christian Theurgy, Druidic magic (English branch only), Islamic magic, Qabbalistic magic, Wiccan magic (Gardnerian branch only), Zoroastrian magic
Shamanism: Aboriginal magic, Aztec magic, Druidic magic (Wild branch only), Shinto, Wiccan magic (some branches)
Traditional Witchcraft: Wiccan magic (all branches)

* Chaos magic is a special case. Due to it’s internal metaparadigm, practitioners of this tradition can work unhindered with members of any tradition regardless of their differences provided the Chaos magician is given some time to learn about the tradition(s) of the other magicians he is working with.

Hey, awesome! I could really use this for my game. Thanks guys.

Also, I'll allow a group to create a group power site in return for a malus on the creation roll. The maximum rating power site that a group can aspect is equal to the penalty they take. So if they want to make a Force 6 powersite, they'll take a -6 to every roll to create the group.

Another idea I came up with was to have it so when people join a group, they have a penalty or bonus based on how many people are from their tradition or a similar tradition. So if a hermetic was joining the Shasta Shamans, he'd have a -1 penalty for every shaman in the group and a +1 bonus for every hermetic.