Chatter, like any social network, is better if more people are using it. That’s why we give every Salesforce customer unlimited free Chatter licenses. It doesn’t matter if you’re a small company with 50 employees or a large, global organization with 500,000 employees. Chatter is free for everyone.

1. Add all employees to Chatter
Use the Data Loader to add all your employees to Chatter at once. To find the Data Loader, click Setup > Administration Setup > Data Management > Data Loader, then follow the instructions.
2. Add new users one at a time
If you want to add employees one at a time, click Setup > Administration Setup > Manage Users > Users > New User
3. Turn on invitations
Turn on Chatter invitations to let employees invite coworkers to join them on Chatter for free. Invitations are limited to the domains you approve. To turn on invitations, click Setup > App Setup > Customize > Chatter > Manage Your Chatter Settings
Chatter Free users can collaborate with other employees in your organization, share documents, join groups, and follow coworkers. Chatter Free users will not be able to see any data updates on opportunities, cases, accounts, and so on.
If you want employees to have more access to Salesforce data, upgrade them to Chatter Plus licenses. Learn more here.