Sometimes, you get jumped by guys and nobody sees you do get beat.
Maybe you are pissed of that they would do something like this, but cannot be convicted because of lack of witness.
Well then let this organization know about your grievance.
If there is others who had the same experience of
the same 'guy' or gang, the private investigator will call you all up. You all can then get the option to pool in the money to pay for the investigation of this suspect.
Now the thing is you can't exactly convict him because he done it long ago, but if he is the type to beat people up, who knows what other secrets he doesn't want the world to know.
The investigator will first ask for everyone to come together for a meeting, to try and remember everything about that guy, and see if he can be found if his identity is not yet known. This shall give the investigator a head start.
If the suspect is found, the investigator will trail him and hopefully find out how he live his life now. If he beats anybody else more, the investigator will now have the video to give to the group. The group can now haul the guy to jail collectively.
But it doesn't have to just stop at this, the information such as him cheating on his wife can do just as well. Or even that he is flouting a bail.
Of course, the investigator might just find out that the suspect is now a nice guy... and the group would just dissolve.