When I add a new item I can add to square sites, and manage inventory, but can't see an option to add to the POS grid if we add new items on the website without going into that section separately,
When you create the new item, scroll down & you will see 'Add to Menus' & you can choose which menu you want the item to go into. I'm not aware of a way to auto-add it to the pos grid.
I'd be happy to jump in with some more information here!
The ability to auto-add a newly created item to your grid on your Restaurants app would be considered a feature request at this time. I'll go ahead and move this post to our Ideate board for Square for Restaurants.