There is safety in numbers, and that is true. That phrase only applies to self defense. When people use the power in numbers phrase, it can refer to the abuse of numbers to do bad deeds, such as jumping an individual with multiple people. This is pure cowardice, and it shows that the individual is too scared to take on the victim by his/her lonesome.
Attacking an unsuspecting victim with multiple people can easily result in massive bodily injury to the victim, and it also shows the intent to do so. Getting revenge on these individuals is generally not looked down upon.