Added a new 5th wire for my wi-fi thermostat, now where to connect it?
You simply have to connect the new wire you ran between the "C" terminal on your thermostat and the "C" terminal on the furnace board.

You can test with your multimeter, you'll get 24VAC between R and C terminals.

Judging from your photo you have an air conditioner (it will be connected to Y and C), and there is already something else on C -- perhaps a humdifier (likely two wires C+W?). 

The only concern here is potentially the load on your transformer. You get maximum of 0.5A at 24VDC (written on the right side in your picture). You'll have to find the current draw of everything, and if the sum exceeds 0.5A, you could run into some troubles (voltage dropping, causing anything from random malfunctions, to everything not working, to burning out your transformer).