Bitrix24 is a free social collaboration platform that comes with unlimited users.
It is also extremely affordable at 99 US dollars for unlimited users. It comes with traditional social collaboration tools, like private social network, group chat,
mobile messaging, project management and videoconferencing, but also features popular business tools, like CRM, workflow automation and email marketing.
Please read Bitrix24 manual, if you'd like to know
how different collaboration work. And if you want to buy on premise editions of Bitrix24 that come with API, CMS and source code, please
download trial version here.