I want to start a network of people in my industry interested in sustainability, so that we can share data, ideas and techniques.

This is what I need the platform to be able to do:

—- I need to be able to invite people individually.

—- Keep the data, and members names out of the public domain. (But a public landing page may be an advantage, which then redirects to a private area)

—- Make it collaborative.

—- Have a way that users can ask a question/partake in a discussion.

—- Not too onerous to sign up, or privacy concerns.

—- Free, or very nearly.

Google docs/Dropbox is nice and simple, but there isn’t a discussion feature, and a folder full of files isn’t very inviting.
I really like the idea of some kind of wiki site, but it needs to be password protected somehow, and I still want the discussion option.
I have some HTML and CSS.

Any ideas?