there's probably a small resistor on the board with that LED.
There is it is 100R
You might also be able to put two leds in series on a single pin
Sorry no you can't there is not enough voltage to turn the LEDs on. You require 3V before you start getting any light out of the LED if you had them in series you would have less than 2V across each LED and it wouldn't light.
you probably can't run two in parallel.
If the supply voltage is 5V and it has a 100R resistor and the data sheet says it has a typical 3v3 turn on voltage then you are dropping 5 - 3.3 = 1.7V. So through 100R this gives you 17mA. So two LEDs would give you 34mA which is just about aright (absolute maximum 40mA). However it would be better if you used one LED per pin. You could then power 8 LEDs at only 136mA, the chips absolute maximum total current is 200mA.