How do I encrypt a bunch of messages by different authors, shared with that group of people?
There are many ways to solve that problem. I would like to tell you my first ideas:

Create a protocol, so that everyone shares the same key. With this key everyone can decrypt the file, write stuff, and the encrypt it again. It is a symmetric approach, but you public key infrastructure may be good to share the key (e.g. Diffie Hellman key exchange. This can also be used for groups). This idea is similar to group chats of messengers. Therefore you could inform yourself, how known messenger apps solve it.

If we assume that they communicate circular ( A -&gt; B -&gt; C -&gt; D -&gt; A -&gt; etc. ) you could only use public key crypto: Just give everyone a public key to decrypt and a private key to encrypt. A writes the file, decrypts it with the public key of B and sends it B. B can encrypt it, write stuff, etc.


What can be a problem is the point that Edgar also wants to know the previous messages. In idea 2. it's simple: Just add him to the circle. For idea 1 it's a bit more complex. When you add Edgar you want to create a new symmetric key. Therefore someone in the remaining network can decrypt the file first, then produce the new key and afterwards share the encrypted file (with new key) again.
Anyway: There are a lot of methods to solve your problem and for most applications performance is very important. Therefore in practice you want to choose the solution which is fastest, cheapest and still provides security.
I hope I way able to help you!