Alright.

Due to the way how archer shoot and the arrows move in, apart from your general, the friendly fire is extremely low.
It depends only on accuracy and till a certain degree on the "speed" of the unit. If we speak about fighting units, there wont be much movement.

A test you can do yourself:

Get you a friend, host a battle and let him take 10 or 19 monks, make large unitsize.
Now you take yourself some archer, in the best case you get different archer with different accuracy.

Now setup the monks very close to each other and make also a bunch with 4-6 units, where you put the units above each other.
Now shoot with the bows and watch it.

I can already tell you the outcome:

1. Accuracy is the way to go, anything below 60 is not a good way, I personal only shot into my own men with accuracy above 80.

2. Distance. If you are too close, the angel of the arrows will be quite low. You need a bigger angel, to not shot into your own people. You want a very high peak and the arrows have to drop down with a high angel. This way the arrows fly above your people.


A Bowcav with accuracy of 80 is a good example, in a cav skirmish the bowcav will kill more enemy horses, than another cav (depends a bit).
The friendly fire wont be too high, actual its quite low with the high accuracy.


Summary.

Good archer seems to be hardcore sniper, they have very low misses and hardly hit different units/mens. To let good bows shot into fighting units is very good.