Homebrew PCBs

What you're looking for is most likely called a "ground plane flood"
and most EDA packages support it.  In the sparkfun link, note that the
GND pin in the lower left is electrically connected to the plane, as
are a few other pins.

In PCB, to get this, draw a rectangle over the board, and use the
thermal tool to connect pins to it, or the 'j' key to join lines to
it.