hey
I think maybe a guy would start a group spell, and it takes a certain time to load up (like 15 sec.) In that time the mage would be harmless and the mages that are around him can do a support spell, so the spell gains in power, thus the more mages, the more powerfull the spell. off course there should be a max for example based on the spellcasters level. The higher the level, the more support it can receive.
When the spell is cast, all the mages that participated in the spell would lose a lot of cast points. But if the main mage gets killed in the mean time, nobody would lose spell points