My players are considering founding a magical group, and are trying to figure out whether or not to try and attract a mentor spirit.
I really like the idea of a mentor spirit for the group -- there's just something about it that seems right.

So... how do you, in your game, run group mentor spirits? The spirit "takes an active interest in the group as a guide and counselor". How often does it pop in to give advice? What kinds of advice does it give -- vague and prophetic ("When the goat turns red strikes true...") or specific and pertinent ("Your Johnson for this run has his own agenda which conflicts with yours")? How much does it know?

we had a group mentor spirit of sorts. he gave us plenty of guidance and advice. mechanics wise he rolled intelligence to see if he knew something i think. or it was handwaved. at times he could be very specific and others vague to the point of uselessness. treat it like divination. its only as useful as you think it should be. also our spirit saved a few of us a handful of times.
all in all if you want you players to know something or to just think they know something those spirits can be fun tools.

I figure that it knows more, the more members there are (and the higher grade they are) in the group.

My query was about whether it's an actual tangible spirit or a vague presence in dreams and visions or whatever.

I suspect, ultimately, the mentor knows as much or as little as you need it to, to make the game more fun.

as far as stats goes we had it as members of the group x2 as the force of the spirit.