So, if a 40m GP guild has 50% sign up, they will be matched against a 20m GP guild that has 100%.
That means average GP of 1.6m for one team vs average GP of 400k for the other.
So you put a 1.6m player's team up against a 400k player, and it will be severely lopsided... And the total number of players is irrelevant since they can't really gang up, all fights are still 1v1.
Basically by only taking in 50% of your guild you screwed the other guild.
I think there should be a second criteria, looking at the average GP of all registered players. If they matched based on average player GP and total GP of all registered players then you would wind up with much less 1-sided matchups...