No Experience Gain While Invisible
No experience gained while invisible would greatly reduce parasitic behavior in mass parties. Now there is no repercussion when someone joins a mass party and stays invisible the entire time. While gaining the rewards without contributing anything.
Perhaps also require an action taken every "x" seconds such as casting a spell on an enemy or ally / attacking something.
What about 50-ish mages who can only invis and are not able to do anything yet in mass parties but would eventually grow to become one of the important shooters that carries the party?
A better idea would be to implement vote-kick for parties.