Hosts create an event and share an invitation link or QR code; guests join from their browser without downloading an app or creating an account. Each person can take a limited number of photos, with no preview or retouching, keeping the experience spontaneous and true to film.

Photos remain hidden from everyone, including the host, until the host chooses to develop the roll. Development can happen immediately, at the end of the event, or the following day, so the images are revealed together as a shared surprise. posa is designed for weddings and also suits birthdays, bachelor and bachelorette parties, trips, and dinners with friends.

The service is free for events of up to five guests. For larger gatherings, hosts select a guest count and pay once per event; there is no subscription, and guests pay nothing.