Even if you did start an HOA, there’s nothing that can force them to opt into the unholy thing.
If they really have 20+ people living in a 3-4 bedroom home, research your city’s/town’s health codes. There’s probably a minimum square footage per person, max people per bathroom, etc.
One could always tip the health department about a vermin problem and let them investigate.
Then again, you chose to live in a non-HOA-infected neighborhood, which comes with many advantages and a few disadvantages. Short of there being an actual health/safety hazard to yourself, I’d do exactly nothing. No point in making enemies or hurting people who are possibly sharing because they’re working hard for little money.
