El Capitan - Where to install application
For command line tools accessed by any user, /usr/local/bin is the best location.

System Wide Requires Admin

That sudo may also be required is also correct. An administrator should be required to make changes that affect other users of a computer. Ideally only users who are administrators can make system wide changes.

Practically this means your Installer package should require admin rights to be installed. Installer.app will take care of enforcement for you.

pkgbuild

Hopefully you are using pkgbuild to create the Installer package.

If so, make sure you pass the destination path /usr/local/bin using the --install-location flag.

Avoid embedding standard parent folders in your package and installing to /. No package should need to install directly to /.

This will ensure any existing /usr/local/bin folder path retains its permissions and your package does not damage any existing settings. If the destination path does not exist, Installer.app will create it for you.