Originally posted by snake IMO, Sigma and Tamron's f2.8 zooms are more usable at 17/18 than the Pentax is at 16. If you want less distortion, then it's Full Frame and the appropriate lenses and switching styles/distances a bit.
Never had an issue using my 16-50 at 16, and surely switching to full frame and a 24-70 or whatever would land you in the same ball park?
It is difficult when you're in a small room and need a very wide group shot. Perhaps try and casually put the less body-image conscious towards the frame edge? Never tried fixing perspective distortion of this sort, let me know how it turns out.