"Sharing" used GPIO pins
No, you can't "share" pins between shields, except in specific circumstances (open-drain interrupts, I2C or SPI buses, etc).  General GPIO pins connect to just one device at a time.

I can't tell from that picture, but it is highly unlikely that all 8 of the IO pins are actually in use by the shield, and the ones that aren't actually connected to anything within the shield are available for another shield placed on top to use.  You'll have to look at the schematic or manual for the shield to know exactly which pins are used for what and which are available for other shields to use.

However, one thing to note with that shield - it doesn't appear to pass through the power header, so how a shield on top would get powered at all is a mystery to me.