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. 

Find out how to invite a team or client
Find out how to create internal posts after you assigned clients
Find out how to create internal notes 

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