How does one get into muck hollow (the portal doesn't open)?

A friend invited me to join them, but I couldn't into the city?
From Balefort castle how do I get into the sewer system?

Thanks

You may need to purchase this dungeon from the Platinum store. Other than that I'm not sure.

Or perhaps they invited a bunch of people and by the time you accepted the party was already full?

I think it was I needed to buy the dungeon.

Once I did, I was able to get there via the world map.