Glycerin can be used as a solvent when making transparent soap, although too much glycerin will make the soap "weepy" in humid weather. Small amounts could be added to regular bar soap, but again there is a risk that the soap will be weepy and too soft. Glycerin can be used to make liquid soap in place of or in addition to water.
But mostly glycerin is a byproduct of soap making, not an ingredient. When you make soap, it naturally contains roughly 8% glycerin by weight.