Pinboards are a shared part of any conversation, which means that all members of the conversation can see all the pinned messages in that conversation. Keep that in mind when pinning things and/or editing the pins on the Pinboard.
If you wish to have some private notes in the form of pinned messages, then simply create them in your Monologue!
Read more about pinned messages on our blog: Pinboards - to pin or not to pin?