As site admin, when you configure the tool in Site administration > Plugins > Manage tools, set the tool configuration usage to 'Do not show; use only when a matching tool URL is entered', then email the tool URL to the specific users.
As site admin, when you configure the tool in Site administration > Plugins > Manage tools, set the tool configuration usage to 'Do not show; use only when a matching tool URL is entered', then email the tool URL to the specific users.