Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
Often times, I want to message more than one den without messaging the whole pack (e.g., send information about a 2-den event). While I can manually select all the relevant people from the second den, it’s somewhat tedious.
I agree this would be a great add to “basic” Scoutbook.
You can do this right now, though, with the Patrol/Den Selection List in the messaging tool of the Feature Assistant Extension for Chrome and Firefox. It’s been very useful for me in the past when patrols wanted to do stuff together.