NB. wrap our data with a meaningless 0 index row for safety!
dat=: 0,.|:0,.|:a$~2$%:#a

NB. score a user row (count of invited users in row)
sc=: 3 : '+/y{dat'

NB. children of a user row (users they invited)
ch=: 3 : 'I.y{dat'

NB. recursively score a row, row + score of children (however deep)
t1=: 3 : 0
	NB. if we have any children
	if. 0<scr"0 y
	NB. sum children scores plus 1 for the child itself
	do. +/1+t1"0 ch"0 y
	NB. if we don't have any children
	else. 0
	end.
)

NB. get our top 3 users
t3=:(i.3){\:~,;t1"0 i.#dat