Photographer Matt Granger (aka ThatNikonGuy) recently published a short but helpful video looking at things to keep in mind when shooting portraiture with an ultra wide angle lens.
Typically, shooting portraiture with an ultra wide angle lens isn’t advisable. The reason has mostly to do with distortion. However, there are some situations where using an ultra wide lens can actually work and environmental portraiture is definitely one of them.
Granger has some useful tips on how to avoid distortion when using a ultra wide angle lens including framing your subject in or near the center of the framer, not shooting to close to your subject, making sure you level the camera as much as possible and leaving enough room in the corners for post processing (automatic or manual distortion correction).