They started in NYC in the 80's I believe.
They do some good work. They hang out in groups. Always in groups in trouble spots and basically make their presence felt. For example they will hang out on a subway platform late at night. They will walk elederly folks to the bank and make sure they get home safe. They will stand on known drug corners and write down plate numbers. They don't carry weapons, arrest people or attack criminals. They do call the police and give statements if they see a crime. They will defend themselves.
Most commuters were happy to see the Guardian Angels standing on the subway platform with them, especially late in the evening.