I'm working on a monome clone with the SparkFun buttons and PCBs. I'm taking a lot out of the Meggy Jr. schematic to drive the LEDs, since my LED grid is essentially the same as the Meggy's. I'm not sure how to add support for an 8x8 button grid though. Could anyone point me in the right direction for adding something like that on? I'm using an Arduino at the heart. I'm pretty new to doing stuff like this, but I'm fascinated by all of it and I'm learning as I go.

What you need depends on, well, what you need.

If you need to be able to accept more than button press simultaneously, then you pretty much need to have a separate I/O line going to each button. You can do this (for example) by using parallel-in serial-out shift register chips to measure the state of each button at a given moment. (Obviously you need well-defined polarity too-- each button connects to ground, and each input line on your PISO chip has a 10k pull-up resistor, for example.)

But, if you don't need multi-press capability, then what you want is a "switch matrix." This works just like an LED matrix, in a sense-- you set up rows and columns and scan them to see which buttons are pressed. Google might be able to help you find some good examples.