Signal is super user friendly. I got my family to switch over years ago and even my mom can manage it. Though, it’s probably a tough sell for a kite flying group more generally. I do think it’s probably a lower barrier than making a new social media account on some fediverse alternative, but hard to explain the advantages to people who don’t care about privacy from Zuck.
it’s a mobile application first, but desktop options are available. Good luck!