I don't think police is usually in these groups; it's just intended as a warning sign for potential, say, burglars, that the people in the neighbourhood are wat

I don't think police is usually in these groups; it's just intended as a warning sign for potential, say, burglars, that the people in the neighbourhood are watching out for each other. In practice, these groups are sometimes used by the racist neighbour to "warn" their neighbours of dark-skinned people walking around. And it's useful for organising neighbourhood events.