Go to Settings > User Roles
Create a new role called "staff".
- tick customer view
- tick website view
- tick job view, edit
- tick job task view, edit, create, delete
- tick "User can login"
- tick any other permissions you would like the staff member to have.
- save the role
Go to Settings > Users
Create a new user
Choose the role "staff"
Assign a password and save the user
This staff member can now login.
If you do not wish for the staff member to see job totals etc.. do not give them any "invoice" permissions.
If you do not wish for the staff member to see any customer contact details, do not give them any "customer" permissions.
Update: if you do not wish the staff to be able to 'edit' job tasks, then you can use this new method of creating staff members. Go to Settings > Advanced and search for 'staff_by_flag' and change this from 0 to 1
Then go to User Roles and for the staff role instead of selecting 'edit' for job tasks just tick the new 'staff member' tickbox on the right.