I generally try to follow these rules :
#1. If it's 1v1 away from a fight, I might hover around the area to watch, and likely vox the friendly pilot asking if they need help. If no answer, or a "no", I won't. If "yes" then I dive on in.
#2. If it's a 1v1 away from a fight, but a squaddie, I'm comin in.
#3. If there are 3 enemies or more, ESPECIALLY if there's only 1 or 2 friendlies chances are I'm joinin the scrum, no notice, no warning.