My understanding is that everything in the Community tab (which is where discussions are) is readable by anyone whether they have joined or not when your group is set to Public. In fact, I am planning a Public community so that content I add will get indexed by search engines and help people to find my community. Then if they want to benefit from being able to comment, ask questions, etc. they would need to join. Since the group will be Public, I will make it clear that there is no expectation of privacy in the discussions. What are you hoping to accomplish by making your community Public vs. it being a Private freemium group?