I tried using the digital pins as grounds (set to INPUT, write pin HIGH) with no luck.
Why? A digital pin that is set up as an input will not act like a ground but will be a high impedance.
There are only three ground pins on the Arduino I'm using. How do I effectively "ground" all five phototransistors so I can get accurate readings from them? There is probably an easy answer to this but I'm new to the world of circuitry.
Choose any one of the Arduino's ground pins and connect that to all the ground connections of your five phototransistors. If you're using solderless breadboard, there are connections along the edges that will do this for you.