Social Collaboration Tools for Enterprise is not only for internal use. They are also meant to help you be more effective when collaborating with your external contacts.
But most vendors make you pay also for external users. When you choose Microsoft Sharepoint or Microsoft Yammer the external users (or you) have to pay for their accounts.
This is not so for the cloud solution IBM SmartCloud for Social Business or the newly released on premise software IBM Connections 5.0. In both these cases external users do not have to buy an account to get access. As a user of either of the two IBM solutions you can invite as many external users as you want into free guest accounts. Giving them access to more or less everything you may want to invite them into.
Have a look at how this works in the newly released IBM Connections 5.0 in this video. Enjoy!