Hi, Natalie!
There are lots of reasons that dogs exhibit selective aggression. Some dogs have triggers (sunglasses, hats, ethnicity, etc.). It could also be an issue of timing, ex: are the visits occurring when your pup has food out? If there are any common threads, this can be helpful in identifying what is upsetting your pup.
Other times, dogs are just protective of "their" space, toys, and humans. Try using some of these techniques to ease your pup's tensions:
-Introduce your pup to any guests outside the home in a neutral space, like courtyard, sidewalk, etc.
-Enter your home with guests. The excitement of you coming in can help negate the anxiety of someone else intruding.
-Introduce your pup to a crate, so that it has a safe space to be while guests are in the home.
-Work on basic obedience with your pup. Basic commands like "sit", "stay", 'down" can be helpful in teaching your pup restraint. They are more likely to listen to a command they know than just repeated pleas to stop a behavior in the moment.
Your dog is small, so your pup's aggression may not be taken as seriously by your friends (as opposed to if you had a larger dog). But aggression can get your dog in a lot of trouble with your landlord, homeowner's insurance, and local authorities. It may be necessary to get your dog a personalized trainer or one-on-one obedience class. But until you get it under control, it is important that you keep your pup separate from all visitors so that they do not continue to exhibit human aggression.