How can I deter my chihuahua/pug mix from barking/growling at visitors in my home. He is loving with us..?

When people come over, our dog barks, and growls, and nips he is loving and cuddly with our immediate family. It doesn't even make a difference when our other dog (jack/pug mix) is friendly and cuddly with anyone who comes in the house.I have tried to have the person offer treats, and he takes the treat but doesn't warm up to others. He is such a great dog with us, and I want others to love him too!