Short answer: No. Sharing someone a section only opens that one section's content for them; they can see the completion status of every other section, but not what's inside. Vision & Leadership can never be shared with anyone.
What a teammate can see
Once you share a section with a teammate, they get view and write access to that section's content. Beyond that:
Every teammate can see the status and completion percentage of all nine sections on the dashboard, including any they cannot open.
Sections they cannot open show as locked, with a CONFIDENTIAL chip.
Vision & Leadership can never be shared with anyone, even if you try. Financials can be shared, but only by a founder, and it is the one section you can cleanly take back later.
Shared reference lists, such as your roles list and document types list, are visible to any member. These aren't section content, just shared taxonomy used across the portal.
What a teammate cannot see
A teammate cannot open the actual content of any section that hasn't been shared with them. They also can't see certain fields even inside a section they hold: for example, base salary and discretionary pay in Your Team stay restricted, regardless of who else has access to that section.
Note: Sharing a section grants view and write together. There's no way to hand someone a section that only lets them submit answers without also letting them see what's already there.
Things to keep in mind
Tip: Share only the sections a teammate actually needs to work on. Apart from Financials, there's no way to later narrow someone's access to a section without removing them from the organisation and re-inviting them.
Warning: The "assigned to [name]" label you see next to a section is temporary and tied to your current session. After a reload or on another device, that label disappears and the row shows your own avatar again, even though the teammate still has access to the section.