A non staff member is a feature in Ultimate Project Manager which allow you to have a member like a contract worker to work on projects, means that the staff is not actually an employee in your company but it’s a freelancer/contractor.
Some area of the CRM PRO is disabled for not staff members.
Here is a list of the disabled features for non staff member.
You can add if you want other Permissions to this staff member like Invoice [VIEW] permission and this staff member will be able to access.
If you need to set a user as non staff member navigate to Setup->Staff open the user and check the checkbox field Not Staff Member