The balance between calmness, and friendliness is a difficult one. Even an excellent obedience dog may not be a good visiting dog if it shows little interest in meeting people. An aloof dog may be calm, but may cause people to feel rejected. A very friendly dog may have the best of intentions but may cause injury. A dog that is full of energy and always ready to work may be too active for most situations.
Any breed of dog can participate.
My visiting dog, Oso , is a large breed terrier mix. I can take credit only for his schooling. The temperament that makes him suitable is natural to him, and the reason I chose to become involved. His girlfriend, Tanith, is also well schooled and well socialized. Her natural exuberance, however, is not well suited to the kind of visits we do.