Logic Level Converter, Arduino and Cellphone
That website has a photo which shows "2-GND". This is the ground that you want to connect to the ground that you have for everything else.

As far as the 2.8v source, you can use any source you want. The voltage is just used to reference the serial signal against. If you tie it to the 3.3V it will probably work. I am not sure of the specifics of that voltage shifter, but I assume it will treat 2.8v on the signal line as being a logic high when comparing to 3.3v.