A grope box is a large piece of furniture found in some SM clubs. It is a large box (either vertical or horizonal) into which a person is placed. Designs vary but often incorporate some form of restraint for the hands and legs to prevent the victim from moving or protecting themselves. The box will have a number of holes along the sides into which people standing outside can insert their hands and play with the victim. The person inside the box cannot see who is groping them, and if their hands are restrained they will be able to do very little about it.
Infernal Mechanix have designed a grope box that uses mirrored glass to allow the people on the outside to see the person locked in the box but prevents the person inside from seeing out.