GMs are very invisible so that would work much anyhow
Which if course is far more invisible say than just regular invisible

I strongly suggest disbanding this guild. I mean, since it's fairly obvious that hunting dragons is out, none of the in-game monsters are really ferocious enough to warrant a whole guild dedicated to their eradication.
I don't think that because hunting dragons won't work in PS's setting that they can't RP that they are hunting dragons. That said I think without anyone else in the entire PS world participating, it would get quite dull and the guild would simply die out. There would be nothing to really do... no reason to even level sword skills because the dragons you are hunting are not really ingame, so all you'd have to do is RP that you are a superior swordsman....It all sounds to me like they need something else to do. A hunters guild might be interesting, but make it sort of mercenary. You could be a guild that can be called upon to hunt down a ferocious ulbernaught, or something of the kind... a tefusang thats been let loose in hydlaa.... or lured there, etc...
No need to tell them to disband... if the idea doesn't work out the guild will disband itself through inactivity. Really though I'd hope they would simply evolve into something more workable.