Now using the word invasion, I mean "big general fights involving everyone".
What I'm recommending, which should generally help in the overall ability to have some proper invasiontype fights is to have a party size cap enforced by temporary scripts which operate during an "invasion period".
As far as I can remember, reading this a month or two ago, player limits can be enforced in parties and therefore can be forced by scripts.
Something like: mark a map as "invasion status" in game, and then when any player joins a party, the onjoin script checks party size and kicks the player if they are in a party that's of say, 5 max.
The thing is, it's been known for a while,(and forgotten a few times) that large parties of players cause lag because of the way data is sent from one player to Ever Other player in the party and thus is an exponential effect on lag compared to party size increase. I don't have the proof at hand at the mo.
Using script capping on party sizes in this situaiton also stops players having to say "please don't make big parties" in ooc constantly, and allows players to use minimaps becaus etheir party only has 5 people in it.
Please remember this is for Invasion situations only, not general quests or anything else.
On one of the first gate invasions I remember getting invited to a party that had 17! people in it. I believe this also happened again when NC was trying to run something at the canal gate.