@wag0325

It really depends on what they’ll be doing.

If they simply need to upload files, such as during the development of a theme, some code, etc, then SFTP would work by providing low-level access to simply use a application to upload.

If they need to install actual software to the server, there’s only two options – root or a sudo user (with sudo being the preferred). I’ll always recommend creating a sudo user and setting it up with both an SSH Key and password (as the password is needed to elevate permissions).

If it’s a different case, please detail what you expect them to be doing and we can provide feedback.