Closing external circuits (Xbox one controllers)

Hi everyone! First off, thank you in advance for any insight you may be able to provide.

I am new to arduino, but not completely new to programming. I'm a sysadmin for a living and just getting into electronics. I also like to modify video game controllers so I'm trying my hand at a project to mix the two.

What I'm looking to do is figure out how I can get the arduino to close an external circuit (a video game controller button).

Part of what I like to do is add buttons to the bottoms of controllers so that you don't have to take your thumb off of the aiming stick to press a button like jump or crouch. The problem with this is that the switch is hard wired to one specific controller button/action.

So I got the idea that I could use a microcontroller and set it up so that I could "remap" the tactile switches I install to any controller button that was connected to the microcontroller....which naturally led me to arduino.

The controller I end up dealing with the most is the Xbox one, which has a common ground setup. The switches I install are just tactile switches with one wire going to the button's (controller ABXY usually) and another to any other common ground point.

The first step that I need to conquer is getting the arduino to "close" the external circuit, just like a switch would. If at all possible, I'd like to do this with the arduino itself. I can fit a nano inside an xbox one controller - which also has 3.3 and 5v points that I can use to power it.

I had found a few resource that said I would need an optocoupler (which Im still not 100% of what its used/required for). - but for the sake of space if it can be avoided that would be cool.

I know there's a product on the market that does this on a single chip that's about 1" square and has an ATtiny328 on it and is relatively thin - but I want to build one myself :)

Thanks again!

TheCultOfKaos: I had found a few resource that said I would need an optocoupler (which Im still not 100% of what its used/required for). - but for the sake of space if it can be avoided that would be cool.

An optocoupler, also known as an optoisolator, is a device with a LED and phototransistor inside. These are completely electrically isolated, the only 'connection' between them is the light. When the LED is on, the phototransistor conducts. They are fairly small, the surface mount type are tiny. http://www.farnell.com/datasheets/1717170.pdf