Group training is only for when all the vaccination boosters are done which is around 6 months old. Group training can be great, again all about homework. Petsmart etc can be okay or it can be horrible. Individual training is the same thing. Gently pushing on the rump when the puppy is calm can work well. Don't say sit until after they are sitting. Dogs of any age can resist the pressure and timing is everything when they are young.