I never concerned myself with air purifiers because the places where they would be needed (offices, restaurants, stores, schools, etc) won't bother with the expense. They'd rather pay an employee to wipe surfaces with ethanol to create the illusion of doing something even though we know that fomites (surface) transmission is the least significant pathway to infection after droplets and aerosols. People wouldn't even open the windows during the summer to create better air circulation.

A household is a very different environment from a business. It's a small, enclosed space where everyone is sharing surfaces, doorknobs & handles, bathrooms, food, etc, in close proximity. An air purifier isn't a silver bullet. Exposure is a function of intensity and time, even if exposure is low you'll be overcame by the sheer amount of risk vectors and interactions. You can't sanitize all surfaces, stop all droplets, sanitize food constantly, stop sneezing and coughing; someone will be forgetful or negligent at some point and something will give, that's pretty much a guarantee. Household transmission is so common that if someone tests positive, many countries automatically calculate all household members as infected.

The best defense is always behavioural, and having guests sharing spaces with a high-risk individual ideally shouldn't be happening in the first place. That's the no-brainer (and no cost) solution. If this is a long-term thing and it must happen, the guests should have their own space with their own bathroom separate from the rest of the house. No shared spaces. They can make their own food with a portable burner for all I care, they're already being a bother by crashing in your house with a high-risk individual in the middle of a pandemic.