It certainly sounds like fear, and growling is definitely a warning that she doesn't like what's happening, which could potentially escalate to biting if her growls aren't taken into account, for instance by continuing to put her in that situation, when she just can't cope with it.
Could you simply find her a place in the house where she can feel safe and relaxed when visitors are with you? A different room with a cosy bed and the radio on and maybe a filled kong or a long lasting chew to help distract her? Chewing releases endorphins, so if you can do something like that she may be able to relax at least a little. It will take the pressure off her and off you too.
I think that having her in the same room as visitors (if I've understood what you've posted) makes her completely unable to cope, and makes a miserable time for all of you I would imagine, and potentially dangerous for your visitors too.
There's no shame in explaining to your visitors that she's nervous and feels safer and happier if she doesn't have to meet them or be near them. One of my two is nervous of people he doesn't know so I put both dogs in a room with a babygate across the door when visitors arrive and then remove the gate once the visitors have settled. My dogs then have a choice, and if the anxious one chooses to stay away from the visitors that's fine by me. It means he's had the choice of where to be and has decided that he's more comfortable away from the visitors, which in turn means he's able to act calmly.