Hi Matthias,
FYI this is one of the most requested features. You can find an extensive update from Coda on this here: Launched: New ways to integrate your toolstack into Coda - #39 by Ayuba_Audu
That topic was for the launch of “full-page embed” functionality. Which could kinda-maybe-but-not-really do what you want. So some people used the opportunity to once again ask about when page/sub-doc sharing will finally arrive.
Codan @Ayuba_Audu was kind enough to write an extensive update on the progress (see my link above).
Later on Coda rolled out another update to the “full-page embed” functionality which made it possible for embedded pages to be editable. You can find more info here: Launched: New ways to integrate your toolstack into Coda - #51 by Gleb_Posobin
So what you are asking for is not possible yet, but the “full-page embed” functionality might be a usable alternative for you: add a custom page to your main doc with just the data you want to share, and then embed that page in another doc. You then give the relevant people access to just that other doc. They can view & edit content on that page without seeing the other pages from your main doc.
Of course there are many downsides to this approach; you’ll need to create a subpage and an additional doc for every user that should not have access to the main doc (=not efficient / a lot of work) AND most importantly your data might not be as secure as you’d think. Let’s say you want user X to only see certain values of a look-up column. You might be able to filter those out of view, but the data is still there “under water”, just not visible to the average user. To quote @Ayuba_Audu:
TLDR; page/sub-doc sharing is not yet possible, but it’s being worked on. Full-page embed might be an alternative for some use cases for the time being.