It's unlikely people can move their friends to their own platform but the best way I have found is to call it a "fall-back" platform for when Discord and others are temporarily offline. Get people used to the idea that is the place to share things they do not want leaked when the big platform 3rd parties expose files. The admin can encrypt the storage and periodically zero out files and zero out empty space for privacy.
People with slightly higher opsec may choose to block mobile proprietary devices.
The ideal hiding space would be visible to the public, and indistinguishable from a mountain of other material that's visible to the public--especially if they don't have to log in to see it. That way if a message is found, the search for co-conspirators doesn't narrow at all.