Anyone may use a grit bin to clear a public path or road, though they are generally not intended for personal use. Typically, a spade or shovel is used to spread a thin layer of grit onto the road surface, covering any snow or ice. The salt lowers the melting point of the snow causing it to melt (see sodium chloride).
As a general rule, yes you can take grit from grit bins assuming that you are using the grit to treat local roads and pavements. Most councils place yellow grit bins around their towns so that, if a gritter cannot clear your road, you can safely do so yourself.