Let people filter posts by category.

Post grid sarch doesn’t have support to add custom input field for search, you can add your own custom input field via action hook at the top of the grid and use these input form data to query post grid item via filter hook, please see this

https://www.pickplugins.com/documentation/post-grid/action-hooks/post_grid_container/
https://www.pickplugins.com/documentation/post-grid/filter-hooks/post_grid_query_args/

Let me know if you need any help.

Regards