We are currently connecting 11 digital using 1 ground pin.
Works fine.
You can actually glue (c/a) the connector together and save pwm cables in the process.
Here is an ascii pictorial of our arrangement. only 4 cables were used instead of 11.
Code:
o|ooo|ooo|ooo|o <- sig
o o
o o <- ground
- -
Hope this helps.
Phil