What's the best iOS app for secure and private messaging with large groups?

Telegram Messenger is totally the way to go for big group messaging on iOS. I've been using it for my local hiking club—we've got over 500 people—and it just works. The secret chats with end-to-end encryption are awesome for sensitive stuff, but even regular group chats are secure with server-client encryption. You can set messages to self-destruct, restrict who can post or add members, and even assign multiple admins so you're not stuck doing everything yourself. Plus, the media sharing is smooth—no more 'failed to send' nonsense when someone shares a trail map or a bunch of photos. Battery drain is minimal, which is huge when you're coordinating events on the go. Honestly, it's the only app I trust for keeping big groups both private and manageable.