How do I do the Partygoer Achievment?

It's a story-related achievement, I believe. Just continue on in the game.

Theres a huge hole some where near the wall on the otherside of the pit with fire, thats where you gotta go.

Yeah this has happened to me too i've completed portal and didn't get the achievement for it either yet although i have the first advanced map achievement which you cant get till after you finish it. im gonna go back through it and try finishing it again and see if that works any one know of any fixes that have been released??

oh and just found this in the FAQ by Ayelis

* Partygoer: Make the correct party escort submission position decision.

You accomplish this achievement in level 19.
After you portal out of the firepit, fling yourself back across the firepit
using a very high portal. The achievement didn't register for me until I picked
up the barrel at the end of the hallway to the left.

ill try it in a bit see if this works

Once again no luck here just tried it might still work for you though

When GLaDOS tells you to lie down and "a party asistant will arrive to escort you to your party" then fling yourself to the ledge.