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It's probably invoked after the fact when a person helps take down a suspect (cop yells "Stop that man!" and a civilian obliges). All the police officer has to say is that the officer requested the person's help and the person is assumed from that moment forward to have been acting in an official capacity as an apparent agent of the police, therefore the DA wouldn't press vigilantism charges and the person would be largely immune from civil suit for injuries as a result of the takedown.

I doubt however that a sheriff would have such insufficient forces in his department that he would FORCE anyone to join a posse. If a criminal group gained such force as to be a law unto itself, the sheriff would call in the feds. It certainly is not an issue in major cities; Here in Dallas they could have 2 dozen officers in full SWAT gear outside your door in 15 minutes if someone calls in a hostage situation.