I did this years ago (in a manner of speaking). We had a small hidden group that wanted to use PGP, which we did. In order for the encrypted text to remain manageable we ALL used the exact FULL PGP key. This meant everyone could read messages and everyone sent encrypted messages ONLY using the public key for that keyset. It was a "jury rig" but it did what we needed. When you have a dozen GPG keys being used the encrypted "volume" gets quite long. This had its flaws, but it had many advantages for our unique group.