Taking pants off with friends

I have a four, almost five year old boy who I, or a neighbor has caught multiple times pulling his pants down with friends. The most recent, him and his friend were drawing on eachothers bums with markers. I have talked to him and threatened to take away toys and ground him, but nothing is working. I've talked with the other moms and they've talked with their boys, but nothing we say is getting to them. Is this normal boy behavior? Why won't they stop?

[RockRose]

They need more supervision.  This isn't out of the realm of boy behavior,  but when they get together,  they need to be supervised.