You're in a large, open room, working side-by-side with someone. A person from the other end of the building comes by and starts casually chatting, initially with just the other person you're working with side-by-side. Someone comes out of an office nearby and joins in the conversation.
Is it rude for you, the other person in the open area, to also join in on the conversation?
Is it rude for you to acknowledge the conversation that had been held out in the open when chatting with the first participant later on?
Is it rude for one of the people involved in the conversation to randomly exclude you even though they were holding the conversation right under your nose?