Chatter Free License: User with Chatter Free License can access people, profiles, groups and files. They can make posts, view comments, upload & view files, join groups. But they can’t interact with Salesforce data such as accounts, contacts or opportunities.
So if you have a requirement of allowing people to only join groups, view/participate in conversation, please allow Chatter Free License to those people.
Chatter External License: Chatter External License allows people outside the organization, such as customer, into private groups in the Chatter community. Users with Chatter External License can –
- see the groups they belong to, and profiles of members of those groups.
- share files common to groups they belong.
- Groups and profiles of the members of groups they belong to.
- Files shared to groups they belong to
- Post to profiles.
- See records or other Salesforce information, even in search results.
- Be followed. Additionally, customer can’t follow people or files also.