Adding columns to a table in Evernote Web can be done with some serious geekery by manipulating the HTML on the page.
Here's how I add or copy rows using Chrome as my browser on Windows:
In Chrome, in Evernote Web, go to the note with the table you want to add to
In Chrome, go to File->More Tools->Developer Tools (the Chrome File menu is the three vertical dots to the far right of the address bar)
Select the square-with-cursor in the top left of the Dev Tools panel (hover text says, "Select an element in the page to inspect it")
click a cell in your table
