When you create a new User / Staff, you will set the Roles. You will be able to create Roles from Settings → Roles
With following permissions checked for each modules your staff / user will be able to-
You want to create a Salesman user and want to give permission for creating customers and invoice but allow only to view the invoices and customers this person created. To achieve this, you will create a Role with unchecked "All data" for the modules Sales, Customers, Companies etc. And assigned this Role to this person.