I want to setup a PRIVATE blog but where all invited can post. Possible?

Secret Facebook group. That's how many a company was founded and many a protest movement was launched!

Try Slack

Does your company already have an existing http://en.wikipedia.org/wiki/Intranet? There are off the shelf software solutions for doing this on an intranet, I know https://wordpress.org/plugins/all-in-one-intranet/ has one and I think https://sites.google.com is basically for this. More professional solutions available for more money.

We don't have an intranet. It's literally about a dozen freelancers spread across the country.

If you're not up to hosting your own, there are lots of https://www.google.com/#q=cloud+intranet products: http://www.igloosoftware.com/, http://www.huddle.com/, http://www.jostle.me/, etc.

My company uses https://basecamp.com/. It's todo-focused (as are most) but has collaborative text documents, discussions, projects and shared calendars. The google term you're looking for is 'https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=collaboration%20software'. You might find http://blog.ghost.org/distributed-team-tools/ and https://zapier.com/blog/how-manage-remote-team/ about tools for distributed teams useful.

My company uses a ning space. We pay for it but it was easy to set up, easy to invite folks, easy to start discussion threads, and intuitive for the users.

https://www.atlassian.com/software/confluence. 10 users is $10/month for them to host it. I can't say I especially like it, but it'll do the job and for corporate software I've definitely been forced to use worse.

How about a private Facebook group? Not visible to the world, only members can see the posts and you can share files and discuss using a tool everyone is probably familiar with.