Three magi use wizard communion to "jointly" cast a level 30+ spell on themselves (D:momentary T:group). The spell is not specifically designed to affect them. How many warping points does each magus gain? Zero, one or two?
I'd say 0, since I'd say that being partly responsible for the casting counts as being responsible for the casting and makes you immune to the warping just as if you had been the sole caster.
However, you could also say 1 - at least part of the casting is not yours, so you are affected as if the whole spell was cast by another.
A nasty storyguide could say 2 - it's the mystical energies of three spells that converge onto you, two of them cast by others. I think this is the least likely - after all Wizard Communion "merges" the spells into one, so to speak.
"The group of magi work together to cast the specified spell throught the unifed power of the Communion." [My boldface, core book italics.]
Seems clear to me that they are all casters, all suffer the same botch chance and roll for twilight, if appropriate, so they get the no warping points from the casting.
And yet, if the spell is not designed with them specifically in mind, they take warping. A spell designed to heal Magus X will not warp Magus X, even if cast by Maga Y. Should the Maga cast it upon herself, she warps. It's a matter of specificity in target, not caster.
"2. Being affected by a powerful mystical effect, unless you created the effect, or it was designed especially for you."
In this case, the casting Maga "created the effect" upon herself, no? So no warping for her. That would also be the case for the group, I tend to think.
Indeed, here's the rub. I also lean towards this view, but I think it's not that clear from the text of Wizard's Communion. The same issue applies to joint casting through the Ceremony ability (infernal, divine, hyperborean etc.). I think the "warping" section could use a single line of clarification.