Ok, I read through Street Magic and I got an idea. Well, it's an old idea (you can find the original thread here) but the rules about magical group helped me develop it a little bit.
I will not rewrite everything of the old post. but I'll include the magical variant in this one.
Magical Group Edge
When a magical group forms, it is possible to magically improve the maximum rating of an existing group if they are already share group edge (follow the link), much like initiation increases the maximum Magicrating of a magician. The cost to "initiate" the group is equal to 10 + (new initiation grade time 3) and must be paid as a whole by the members of the group. It means you divide the karma cost by the number of individual in the group (rounded up) and this is the value everyone has to pay for the group to "initiate".
Whenever a group initiates, he may choose the following metamagic abilities.
* Group Centering
When making a ritual spellcasting check involving members of the group, every member is given a bonus to his drain check equal to the group's initiation
* Group Signature
The group may have an influence over it's astral signature just like a magician wiith the flexible signature metamagic.
* Group Masking
The group may have an influence over it's astral signature just like a magician wiith the masking metamagic.
Any ideas/comments, etc. are welcome.