There’s no way to sort by rank in the Send Message interface.
One option would be to install and use the Feature Assistant Extension for Firefox and Chrome to create a custom group. First, create a rank report (or otherwise determine the rank of the scouts). Use that list to select the relevant scouts in the Send Message interface and use the extension to save the group. It only saves locally, so you’ll have to copy the saved list to any other computers you use. It’s static, so you’ll have to actively maintain the list as scouts change ranks.
https://discussions.scouting.org/t/feature-assistant-what-is-it/98647
