A manager or co-manager can choose to include or exclude certain members of the team or organization on a workstream. This will allow your team to share work more easily and collaborate on cross-functional projects without creating extra teams. Click on the kebab icon in the upper right of the workstream page and choose User Permissions. A window will pop up showing you who is currently able to access the workstream, and you can add or remove people from this list.
When you share a workstream with someone outside of the workstream's team, that workstream will show up under their Shared workstreams on the Boards dashboard and left pane.
If you don't have permission to edit the visibility of a workstream, you can always double-check who can see a particular workstream by clicking Contributors inside the workstream's dashboard above the list of action items.