How to add additional I2C sensor module on Arduino
The GY-521 sensor breakout is an I2C slave device. As long as you can obtain (or configure) the additional sensors as I2C slaves with different slave addresses, they would be controlled the same way as the first one, other than having separate communication initiation for each of the sensors, and then reading the data back from each sensor by its I2C address. 

Unfortunately, the MPU-6050, the Motion Processing Unit IC in the GY-521, supports only two I2C slave addresses, so at best you would be able to find (or modify) break-out boards to support these two addresses (by the logic level applied to pin AD0 / pin 9 of the IC). If you actually need more than 2 devices at the same time, look at other similar devices and their corresponding breakout boards.

The physical wiring involves all the modules being connected in parallel on the same pins you have used already to control the first module, also known as an I2C bus. This would only work if only one of the modules has its pull-up resistors enabled. On the schematic for the module, these pull-up resistors are marked R4 and R5, 4.7 kiloOhms each. 

On the GY-521 board shown in the link provided in the question, the pull-up resistors appear to be 2.2 k each, as seen below:



The actual board you have would need to be examined, to verify which specific little SMD parts are the pull-up resistors on it: Not too difficult to do, since they are tied to the SDA and SCL header pin pads. 

On all but one of the sensor boards, you would need to de-solder those two resistors carefully, without messing with any other parts or other solder junctions on the board. If you are not accustomed to working with SMD components, it might be better to start by practicing on some scrap boards salvaged from condemned printers or other hardware, first.



It is a bit much to ask someone to write the code for you, but there are several Arduino tutorials on communicating with multiple I2C devices sharing a bus. Search on the same site you found the guide for this module.