I'd recommend checking out Element, it's free/libre and open source, it can be self-hosted (so no fears of censorship), doesn't collect data, and it's part of the Matrix network, so you can communicate with anyone else who has an account on the Matrix network, even if their account was registered on a different server.
Actually, there is a guidebook:
https://www.amazon.com/Business-Plan-New-Political-Party-ebook/dp/B073Z35SNW
I recommend it to everyone who has an interest in starting new political parties or growing minor parties. It’s certainly helped the American Solidarity Party get its act together, and I know a few Forward Party folks who endorse it as well.