Invision has a (low-security) encryption feature. To make use of it, all senders/receivers will need to be using Invision. Go into the Invision menu, Main Settings, General tab. In the top right is the encryption/decryption key. Set a key there, then share that key through other (preferably secure) means to all parties. Once everyone has the key set, use
/' or
/e followed by a space followed by your text, and it will be sent encrypted. This works in channels as well as PMs/queries (although in queries, the decrypted text for other users will appear in their status window).
For higher security, you'll need something like FiSH -
https://syndicode.org/fish_10/ - read the readme as well. Note that both/all users will need this in order to make use of it.