In order for your staff to be able to complete tasks and access various sections of the site, you will need to create accounts for them. It's easiest to do this when adding a staff member, by simply filling in the "Login Information" section of the "Add Staff" form.
But you can also create a staff account after the staff member has been added. To do this, go to the staff list under the "Employees" tab. Click on the row of the staff member for whom you want to create an account. In the "Login Information" section in the right column, give the staff member a username and password. You can also review their account permissions and control which parts of the account they can access. For most sections you can choose between 1) no access, 2) view and 3) edit. Submit the form and they can now log in with the username and password you created.