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:

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