Best multi-user business software?

I'm looking for a multi-user business software preferable freeware that will work on one PC for up to approx 15-20 users. That offers the user to log-in and see what tasks or ignments have been igned to them and possible let them confirm that they have completed it. That will also allow all users to send messages to other users & for everyone to see what updates or changes that will be happening. I know this is a big wish list, don't know if there is any software out there that offers all of this. Thanks in advance for any suggestions as I'm trying to go as paperless as possible.