This feels like a common courtesy I don’t see enough, particularly the second point. In a conversation circle, always try to keep your side vision open to people trying to find their way in. It can be awkward to try and find your own way in so giving them a spot tells them they’re included then drop a quick brief on the conversation “We’re talking about pineapples on pizza, good or bad” gives them context to jump in immediately.
How do I invite them in though?
Look at them, tilt your head up, and at the next pause give quick context. Maybe take a step back if they’re next to you and the circle is closed.