You have to use a Grafana Admin user to create users and the authentication has to be Basic Auth. (I updated the docs to be clearer on that point.
curl -XPOST -H "Content-Type: application/json" -d '{
"name":"User",
"email":"user@graf.com",
"login":"user",
"password":"userpassword"
}' http://admin:admin@community.grafana.com/api/admin/users