Ars Magica generally makes it easy to figure out what effects stack and which do not, but I have a question regarding this Maleficia Incantation/Consumption guideline:
General: Give a target a bonus to rolls for a particular Ability equal to the magnitude of the effect. This also penalizes a victim’s rolls by the same amount.
What happens if this is cast multiple times against different targets, or if a group version is used? Would 5 targets, for example, each have a -1 penalty to brawl, while the caster would end up with a +5 bonus?
The penalty applies to a target. In this case the caster- group target wouldn't be needed since the people effected aren't the target or casted against.
Strictly speaking in terms of RAW, only one of the group would suffer at a time mechanically.
How I'd do it is if the group is acting as well, a combat group the group as a whole is suffering the penalty. If each individual is acting on their own, its whoever the caster is acting against- so basically any contested rolls like attack vs defense.
Even for the group, that roll represent 5 concurrent hits from the 5 members. It seems obvious each -1 applies to a separate attack and could not stack.
You could have an argument that it reduces the trained bonus from Leadership, but nothing else.