What service is best for in-app video conferencing

We're building video chat into our iOS app, and we're evaluating different providers. The main requirements are below, but generally, these "video conferences" should act and feel like rooms - i.e. it's easy to enter, easy to leave, no one has to set it up, when everyone leaves the room, it's still there, etc. For our project, the requirements are: - Users can join and leave when they want. Some services only let you join the call from the start. - at least 9 users on a call - iOS