All of your users have two types of memberships in your workspaces - Team or Client/External member. When inviting or selecting a user as "Client/External" you'll have the option to create internal notes and posts. It will allow you to create posts & comments only visible to your team.
You can assign team membership when inviting a user.
Team members can view both internal and external posts and comments.
Client/External members can only see external posts and comments.
Additionally, you can change a user's membership later on from the members settings: