Setup –> Chatter –> Settings –> Edit –> Enable Unlisted Groups.
Unlisted groups are similar to private groups in that only members can view an unlisted group’s feed or files. However, unlisted groups provide more privacy compared to private groups.
1. Membership for unlisted groups is by invitation only. Nonmembers can’t ask to join them.
Unlisted groups don’t display in list views, feeds, and search results for nonmembers. Only members and users with the “Manage Unlisted Groups” permission can find and access an unlisted group.
2. You can’t see unlisted groups on a user’s profile in Chatter unless you have access to the group.
Even users with the “Modify All Data” or “View All Data” permissions can’t access an unlisted group unless they’re members or they have the “Manage Unlisted Groups” permission.
Nonmembers can’t visit the group detail page. (With private groups, nonmembers can see a truncated version of the detail page, which shows the name, description, and member list, but not the feed or files.)
3. Chatter enforces group name uniqueness across public, private, and unlisted groups. If a user tries to create a group with the same name as an unlisted group, they’ll see an error message that informs them that a group by the same name exists. For this reason, we recommend that you don’t include sensitive information in unlisted group names. Choose names that are difficult to guess or not obvious. For example, don’t use an obvious name like Acme Merger.
Limitations:
If you decide to create an unlisted group, make sure that you consider these limitations.
1. Custom pages or third-party applications integrated with Salesforce could expose unlisted group information to users who don’t have access via the Salesforce UI. Check in with your administrator about who can access information in unlisted groups in your organization.
2. After creating an unlisted group, you can’t change it to a private or public group. You also can’t change a private or public group to an unlisted group.
3. You can’t use topics in unlisted groups. You can add a hashtag topic when writing a post or comment in an unlisted group, and the topic will be formatted as a link after you post. However, a topic detail page isn’t created, and the link won’t work.
4. You can’t mention unlisted groups in posts or comments.
Cheers!!!