I think you need to speak to the consultant first - they used to allow it at my group when I went a couple of years ago but obviously you'd need to let them know your work pattern in advance and whatnot.
Best ringing before going in to join maybe? I think there is usually a contact number for the consultant if you do a groups near you search on the SW website
One of my friends is a shift worker and as long as the consultant knew this, she was fine with it and didnt charge him back fees. Its better to explain it to the consultant who can then give you the support that you need in another way.