Like Yoshi-P, I think optimizing parties as a group of four is pretty swell -- a tribute to the maximum party size of Final Fantasy IX. Doubling that size for tougher, larger scale events seem all right to me (in this game). Other than behest, much of the content that I have seen look well built and accomplish-able on groups of eight or less.

Of course, finding that optimum ideal number for party and alliance play will depend largely on the subjective variables of the individual gamer.