Don’t do this on a hard floor and be aware of any who have weak backs. There’s a danger of falling so consider doing this where this soft ground to land on, like sand or crash mats.
The challenge is to try to create a pyramid of people, by building up layers of individuals resting on all fours (hands and knees) on the backs of the players below. E.g. with 3 people have 2 on the bottom and the third person resting on their backs. With 6 have 3 on the base, 2 on the next layer and 1 on the top. If you want to keep the height down, just limit it to a certain height of people.
It’s not as easy as it sounds and very prone to individuals falling or the pile collapsing.