Examples of prey predator interactions where density of predator is very low and prey form groups when the predator attacks the prey for food
Many species of animals use flocking/ schooling/ shoaling/ etc as a mechanism to confuse predators. A few examples are tuna, starlings, goldband fusiliers. tuna and goldband fusiliers use their reflective scales to blind and confuse the predator, whereas starlings use sheer numbers to overpower and throw off their predator. The large groups tend to scare away far away predators.

The act of grouping is not dependent on whether or not the predator is endangered or not, but there would be cases where both the predator is endangered and the prey uses grouping. For example, several species of owl are endangered, which prey on starlings. 

https://en.wikipedia.org/wiki/Collective_animal_behavior#Protection_from_predators

http://www.fcps.edu/islandcreekes/ecology/european_starling.htm

http://rsif.royalsocietypublishing.org/content/10/85/20130305