The table for creating a new magic group includes a modifier for using an actual Mentor Spirit as you groups Avatar. This is a fine and good concept to me and I have two magic groups in my current game that have done just this (Dragonslayer and Seductress in case you're wondering). What if any game effect should this have?

I can think of four ideas,

1) Nothing other than kudos.
2) The group gets any bonuses on applicable during group only rituals.
3) All group members get the Mentor Spirit on top of anything they have already (grim potentially)
4) All group members get half the bonuses (ie +1 to stuff) and the full penelty (maybe increasing the threshold for members who already have that Mentor Spirit).

I'm unsure how to handle this but think that it should do something.

I shouldn't have thought that magicians who already have a mentor spirit should be permitted to get a second one.

I don't have a great problem with people in the group getting a mentor spirit through a magic group.

If you were, as the GM, to allow a PC to buy a quality at a later stage it would cost 10 Karma, I think (it is double the cost of the quality to buy off a -ve quality, so in the unlikely event that you were to allow purchasing of +ve qualities, the same cost would apply).

Whether you make the PC buy the quality or give it to them for free is up to you, but if they do want it they must take it at the time of joining, and should they leave the group they lose the mentor spirit.

Street Magic (pg. 65) says that a mentor spirit as avatar doesn't grant any bonuses but will offer advice and possibly assistance on metaplanar quests. I don't see any necessity to go beyond this (and certainly not for free), but I like Ornot's idea of letting members puchase the mentor quality after joining if they don't already have one. It would depend slightly on flavour, however. I see shamanic totems as something you get right at the beginning, but I could see some faustian spirit for a hermetic circle or black magic cabal being obtainable later on.

-K.