Calibre has a builtin server that can allow you to share your ebooks over the internet.
From the user manual:
Start Content server: Starts calibre’s built-in web server. When
started, your calibre library will be accessible via a web browser
from the Internet (if you choose). You can configure how the web
server is accessed by setting preferences at
Sharing over the net
Also check this question.
Of course, you are responsible for every copyright issue that might arise, this is just a technical way to perform a task; it is your responsibility to check if you have the rights to share those books with other people.
This way, you can share the ebook files, that every user will have to download and read with his own device/software. A direct tighter collaboration like you are asking (i.e. shared annotations and comments) is not possible with this method, every user will be reading on his own device with a different copy of the ebook file, but you can use some free online tool like Google Keep, Simplenote, Trello, or something similar to set up a workspace to share notes and thoughts, if you take care to reference the ebook/chapter/section you are discussing about.
Another possible solution is to use a Social Network dedicated to books, like Goodreads, and create a private group to discuss