What group of bacteria normally doesn’t have cell walls and how do they maintain themselves?

Bacteria that belong to the Mycoplasma group don’t have cell walls. To make up for the absence of a cell wall, Mycoplasma has exceptionally tough cytoplasmic membranes that are highly resistant to rupturing. The durability of the cytoplasmic membrane is due to the presence of sterols, which are formed from cholesterol that mycoplasma acquires from the environment. The sterols increase the forces that hold the membrane together, contributing to their toughness and higher resistance to rupturing. 

Mycoplasma have a parasitic or saprophytic lifestyle, which allows them to survive without a cell wall. They typically grow inside their hosts in the controlled osmotic environment of the host cell. They are killed quickly when placed in environments that have very low or very high concentrations of salt.