Why is my cat growling at strange people she doesn't know?

It can be something as simple as your friends with another scent of an animal on them,cats have amazing sense of smell so pick up scents so quickly,so it could be they have been close to a dog or another cat before coming to your home and she is defending her home from the strange scents,

for example your friends may have walked where another animal has so scent has transferred onto them,your cat then feels the other animal is close by so gets defencive about it as her home is her territory.

Just as we grow up from being adolescents to teenagers and then adult. Your cat is more than a year old. It means its a teenager in human years. There is change in hormones and behaviour in this phase. Since your cat is growing up, she is behaving differently. Its absolutely normal. Do not worry. She will get used to the new guests and people around.

she is probably growing out of her kitten phase into a cat and because she is not used to people coming into her territory she is trying to warning them off. This is my home go away!. if you have more regular visitors she will probably stop.

one of my cats does this. well he tries to hiss but like he meow it is so quiet you can barely hear it.

sounds like somewhere something or someone has scared her and now she is on her guard all the time, especially is she is used to a quiet house, people coming in are invading her territory

Maybe has a bad sense or something.