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Share a Chatter Post with a Chatter Group
You can share posts with public Chatter groups and groups that you’re a member of. You can’t share a post to a private Chatter group that includes external users. You can share public posts, but you can’t share feed tracked items and custom feed items, like approvals, dashboard snapshots, or case interactions. You can’t share posts with archived or unlisted groups or private groups that include external users. A post’s comments aren’t shared. But the shared post links back to the original, where comments are available. In Salesforce Classic, files attached to the shared post are included, but in Lightning Experience, attached files aren’t shared.

