To begin with, there should be a change in mob spamming rate:
The more people are in the same place, the faster the rate, for example: if theres only one person in the room the mob spawn rate would be 1mob/5secs, but if there were two players in there, it would be 1mob/4secs and so on. (If a character doesn't move for 10-20 seconds straight it stops increasing the spawn rate, so people cant abuse this system)
-This would not only make it worth it to level with other guild members as it would also make it possible to increase the number of players that could be playing in the same room, witch would also mean an increase of the maximum number of players.
There should also be a bonus xp when leveling with guild members (right now, if I try to level with a guild member, i take about twice as much time to level up, as i would if i were alone).
I recommend about 1.2x to 1.5x whether you do or you don't do the increase mob spawn rate change.
Finally there is one major change, this would take a long time and it is just an idea: dungeons!
I'm sure you've already gave a lot of thought about this and you probably have a good motive to not have done them yet, but it would be so awesome to have your own private dimension where you could farm and level with your guild members without the usual and very boring competition with the other 200's players that also want to do the same!
There should be an item required to enter each dungeon, like a pass that would be spent upon entering it, this item would be only dropped, not bought. This way people couldnt be all the time in dungeons, because they would have to farm outside first!
Sorry for the long post, and keep it up Rob!