The party system works like this:

Same classes in the party:

2 characters in the group receive 95% experience each.

3 characters of the same class receive 90% experience each.

4 characters of the same class receive 85% experience.

5 characters of the same class receive 80% experience.

6 characters of the same class receive 75% experience.

Party Bonus:

2 characters of different classes receive 105% experience each

3 characters of different classes receive 110% experience each

4 characters of different classes receive 115% experience each.

5 characters of different classes receive 120% experience each

6 characters of different classes receive 125% experience each

This formula concerns ideal conditions, i.e. all characters are of the same level; At different levels, characters with a higher level will receive more experience.