Focus on the big picture, rather than just your current post. In Social Media Management, you can use Instagram Grid View to see how your new post will look alongside existing posts and those which...
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It may still be intended as a metaphor rather that a literally reference to their Instagram account.
A grid is a network comprising infinite parts in various locations, as in the utility grids.
I think in this context it means whatever little space in a vast network.
She said that she's already shared her top 12 books in a podcast. She now wants to tell her followers about some more books she loves. If you imagine her original list of 12 books as a "grid" (like the screenshot in post #6), I believe she's now saying she wants to add some more books to that "grid".