it kinda seems everyone can contribute, and the more energy it has, the more powerful it\'ll be...
Yes, and the more powerful it is, the more powerful you need to be to contain it, magic isn\'t a toy.
but I think that would hurt small guilds too much, as their guild-level spellcasting would be highly dependant on the number of mages they have, and only as powerful as the weakest.
But small guild would probably only go to war with other small guilds, and larger guilds could crush them with superior numbers, besides there will probably be more magicians of low level in a large guild, therefore limiting how strong spells they could cast. (note: it would only limit how much energy they could throw in the spell, not the spell level. So two mages with azure way 8 for an example could either team up and use all of their magic points together or surround themself with weaker magicians and throw one level 8 spell each, the power they lay in it would be equal to the lowest magic point multiplied with the number of participants.)
I am also agreeing with your idea of minimum number of participants in a ritual, it would force guilds to let lower level characters to join in a ritual.
And one note more, I think that it would require at least half an hour (ingame) to ready all things needed for an ritual (pentagrams for defensive spells and maybe animal sacrifices for offensive and so on.)