All your conversations are separate, even if some of the same people are in both conversations. For example, if you have an ongoing conversation with just Jane, and then Jane is included in a different conversation among a larger group of friends, the two conversations are completely separate. No one else can view your conversation history with Jane.
Please be aware that while all conversations are private, they can be forwarded, much like traditional email. Also, new friends can be added to group conversations, which allows them to see all previous messages in the conversation.