Essentially, it was believed that party size was related to the amount of EXP you gained per monster kill. Not only that, but AI characters also counted towards this EXP bonus. A very easy one to test!
For the test, I slaughtered Set Sadinians on Very Hard in the Beserk Dragonkin Arks Quest Starting with just a single-member party. Upon adding a Friend Partner to my party..
The EXP does indeed go up, really confirming the rumour there and then. Maxing out the party with Friend Partners raises the EXP to this:
That’s a significantly higher amount of EXP than you get for going in with no AI characters, in the region of 20% more.
You may be aware that, generally speaking, inviting AI characters to your party is frowned upon in the player base, especially if you don’t lock your multi-party. It does after-all deny actual players a spot in your party. This is a bad thing because while having AI characters in your party does increase the EXP per kill as well as the complimentary benefit of increasing the size of enemy spawns it doesn’t increase spawns as much as an actual player would. Therefore you are ruining the multi-party’s potential for all players on the map.
Incidentally, the effect does not include players or AI characters from other parties, it only scales from your own party. This means that the most effective way to grind in a multi-party area is to join as a full party, but don’t do it with AI characters
Seems to be a self-defeating system in that respect doesn’t it? A player gains a fair advantage bringing AI characters with them at the expense of all other players in the multi-party.
Overall, it just seems like a clumsy way to encourage people to play together and yet another mechanic in this game seemingly engineered to punish players for soloing.