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.
Low level mages can still cast paralyze, great heal, fire-strike, etc. They are able to help, and being in the back not casting the highest damage spells are unlikely to be targeted by mobs.