I am using a Sainsmart 8 channel solid state relay wired as shown in this image (which I copied from this question) to control my sprinkling system.
If you look closely at the image of the Sainsmart relay board, you will see there are two ground connections: one next to the eight input connections and one next to VCC. Why are there two ground connections? Should I connect both of the grounds to ground GPIO pins on the Pi?
Think of it that way: if you were to power the relay board not from the Pi but from a separate power supply you'd want to have a VCC and a GND pin on the board to connect it to. The other GND pin at the digital in connector would then be connected straight to the Pi. It's simply convenient not to put more than one wire into each connector slot. Both GND pins are connected on the PCB of the relay board.
As in Ghanima's answer the 2 are almost certainly connected together.
There are very good reasons for separating power and signal grounds in many devices - specifically minimising crosstalk and voltage drop.
If you are powering the board from the Pi over relatively short connections neither of these would be of concern. Just use a single Ground.
