We recommend adding them as investigators since they are not admin users or clients.
If you would want them to have different access from your regular investigators, you can create an investigator role specifically for contractors. You can do this by following the steps below:
1) Go to "Settings" in the left-hand navigation bar, then click "Roles & Access" under Users & Security
2) Click the "+" next to the Investigator tab, then name the role "Contractor" if you would like, then click "Add New".
3) Adjust module access for the new role by clicking the access level under the name you chose for the role