A collaborator post appears on both profiles and shares likes and comments. In the composer you can add up to 3 collaborators by their Instagram username (not available on Stories).
The invitation step people miss
When your post publishes, Instagram sends each collaborator an invitation inside the Instagram app. The post shows on their profile only after they accept. Until then the collaboration is Pending - this is normal and not an error.
So if a client says "I don't see the post on my profile":
- Ask them to open Instagram → notifications → collaboration invite.
- Once they tap Accept, the post appears on their profile too.
The invitation can also be declined or simply expire unnoticed, so it's worth telling collaborators to expect it.