In reply to athickett:
I was going to recommend Moel Siabod as well - up through the old quarries to Ddaer Ddu and along the quieter NE ridge before dropping back down to the track across the moor. Alternatively there is a dull but easy path down to Plas y Brenin which can take you to the Bryn Tyrch.... Some good large houses in Betws y Coed would sleep 15 or so, or if you just needed space for 6 there's the one by the car park at Cyfyng which is both well placed for the hills and the pub afterwards, or the 3 cottages actually at the base of the track across the moor.
Have fun - my stag do ended up a fellrunning trip from Brecon to Pen y Fan and back before curry, beer and a band. All very enjoyable and without the pressure to be 'crazee mad' that can plague stag dos.
Cheers
b