Is there a way to import or copy layers and groups from one Krita document to another?
Not just as a flat image layer but with child layers, masks, filters etc. intact.

Hi animationista,

make sure you have sub-window mode active ( not tabbed multiview) this can be changed from configure krita dialog.
Open both document side by side, then Drag the layers and group ( whatever you want) and drop them in the layer stack of the destination document.

I know this drag and drop is a bit long way for tabbed mode users, i have already filed a wish bug for that
you can subscribe to that wish bug here -> https://bugs.kde.org/show_bug.cgi?id=345449

Thank you very much!

I tried dragging but only in tabbed mode...
I hope the bug gets resolved, it's a bit hidden and awkward to use as you point out in the bug report.