With Magestore POS, the POS staff account will be separated with admin account (which can be used to access Shopify admin).
Go to Magestore POS → Staffs → Create staff
You can fill in the staff username, password, display name, and email. You also need to select the role (if you have not set up the role, read it here How to manage POS role in your store) for the staff, and also the location (if you have not set up the location, read it here https://help.shopify.com/en/manual/locations/setting-up-your-locations) this staff account can log in.
Then you can click on Save button, the saved staff can be managed in the Manage Staff page.