Nestling Array in PHP from MySql

Well i need a little ideal on nestling this php array.

This what i want to do.

Have you heard of online clubing? http://www.disneytreasures.biz/

This how it works,

1) (*Agent*) You bring 3 persons (first you as an agent or officer)

2) The 3 person you brought, brings 3 persons each

3) Again each persons under those 3 persons bring 3 persons again, on and on

So, how can i caculate the total persons under you (*the Agent)*

i want to use MySql database, or can you help me with the table structure?

The ideal is like Clubfreedom. and if you see any tutorial on this, please
help me with the link.

Any ideal will be helpful.