I added them as an investigator, but wanted to see if there was a better way.
Adding them as an investigator is the correct way to add them, since they are not admin users or clients.
You can assign them a different role, if you want their access different then your investigators. You can do this by going to settings>Role & Access.
Once on the role and access page, you will click on the plus sign next to "investigator role" and then label the role. You can use "Contractor" if you want. Next you will click on the icon for manage access and choose what you want them to see.