All users have the ability to start a conversation and select who they want to view it. Only those people added to the conversation will have access to view, edit, or reply to it. This does not apply to admins (admins can view all conversations).
How do I see who has access to a conversation?
Simply click on a conversation and you will see everyone on the conversation on the top right of the screen. If everyone on the project is included on the conversation, you'll see the following icon:
Who can change permissions on a conversation?
If you are a project manager on a specific project and included on a conversation or you created the discussion, click on the conversation and make the necessary updates by clicking the + button on the top right of the page.