I have green striped mushrooms and brown mushrooms. From observing mine, the new ones appear under the existing ones. The new ones then move slowly out from under the parent. Others may have a more biological explaination of how it actually happens.
Salty Rick...that is exactly what mine do. I have many different colors of shrooms and they all do it the same way.
Sometimes the parent mushroom will migrate and leave babies behind. Looks like snot on the rock until the heads start to form.