Why not give the party leader an option to extend the max party size to accomodate extra players? this way, people would neither be left out, nor would there be any pressure to get a full party together because it would be purely optional.
In order to keep the balance, extra mobs would spawn for parties <=2 extra members over the regular party size, and even more mobs would spawn for parties <=4 members over the regular party size. Players would be warned about the extra mobs when extending the party.
Max extended party size would probably be around 12. Perhaps if people felt this would knock the balance of the game, this option could be limited to special areas such as sorrow's furnace, uw and fow only.
This would balance party creation nicely as <= 12 players would be catered for by extending the party, whereas >12 players could split into 2 nearly full groups (6 and 7). Currently, with 10 players you have to split into 2 small groups of 5 or 2 people are left out.
Anyway, to wrap it up, the benefits are:
- More flexible party sizes to accomodate medium/large guilds
- Extra mobs mean difficulty/balance is retained
- If there are too many people to fit into an extended party, two regular parties can be formed instead.
- No advantage to creating extended party so no pressure.
- Greater sense of scale - bigger parties could be fun!
WRONG