Do I need to pay for collaborators?

re: 1. If I want to add employee to the workspace, I need to pay for his user aswell?

If it's read-only it'll be free, anything else will be charged.  Airtable has documentation on this found here: https://support.airtable.com/docs/how-collaborators-impact-billing
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re: 2. If I want to hide from the employee certain tables, views and dashboards, is it something possible to do with airtable?

The recommended way to do this is with Interfaces where you can create pages with specific data visible, and only give users access to those Interfaces without access to the underlying base

https://support.airtable.com/docs/managing-and-sharing-interfaces

I still dont understand.

1. How can i hide bases from employee?

2. How can I hide dashboard from employee?

3. How can i hide views from employee?

If someone has access to the full base, you cannot hide anything from that person. However as Adam pointed out, if you share only an interface with the person, that employee will be able to see only the interface. 

If you want to hide a base, make sure you do not add the user to the workspace. Only share the interface with the employee, not the workspace, and not the base. 

So its not possible to hide base from employee?

It is possible go hide a base from an employee, as long as you do not share the workspace or the base with the employee. 

So for example if i have 5 bases i can share with the employee 3 and he wont be able to see the other 2 bases?

Yes