This will forestall any misunderstandings if you buy the house and commissions are divvied up. Attend during the hours specified. Agents and homeowners don't appreciate stragglers and early birds. Don't be offended if you're asked to register or show some identification. Agents who conduct open houses alone are sometimes the targets of crime.
Most open houses have some form of screening.
Get a copy of the listing if you're considering the house. Houses become a blur when you visit one after another. A recap of specifics, which usually comes with a photo, will help you remember the best ones. If a listing isn't available, take notes. Don't bring a crowd.