Group that includes animals with backbones?
The group that includes animals with backbones are vertebrates.
Sand dollars has a rigid skeleton known as a "test", but these are not backbones so they are invertebrates. Vertebrates are animals that have spinal chords, heads and backbones. Sand dollars are animals related to sea urchins, in a different group called Echinoids. These have none of spinal chords, heads or backbones, so cannot be vertebrates.