All of your users have two types of memberships in your workspaces - Team or Client. When inviting or selecting a user as "Client" you'll have the option to create internal notes and posts, here's more about that. It will allow you to create posts & comments not visible for your Clients.
You can assign team membership when inviting a user.
Team can view internal and external posts and comments.
Client can only see external posts and comments.
Additionally, you can set user as team & client in the 3 dots options menu, here's how.