{Hmmm.} What if we had it where a high level player could help a low level party do low lvl content like Toto-Rak but in a support role? kinda like the battlewarden or the guy on the skirmish islands (thinking it would be like signing up with your GC to 'oversee a party of new recruits whilst they exterminate NM X in X dungeon), only being able to use special GC buff items (like a level-scaled regen or something, or maybe make it where having an 'officer oversee' gives your pt a small xp boost, like 10-15%) and giving advice to the party. Let the party vote on if they need a 'GC mentor' or not before they enter so people can get the experience of the 'unknown' if they want.

As for getting the right amount of lowbies to do said lowbie dungeon, have it set up through the GC, like if you have it flagged you are automatically set up in a queue of people who need it, and would recieve a tell from a GC linkpearl that says a full party is ready to undertake x mission, would you like to join? or w/e, of course maybe not like that but you know...
Of course thats all assuming that dungeons and other stuff would be set up through your Grand Company :/