on *:PART:#:{ If ($nick($chan,0) < %minUsers) { part $chan Channel Does Not meet Required Users Of %minUsers   } } 

on *:START:{ set %minUsers 3 } 

on 1:INVITE:#:{ 
  join $chan 
  timer $+ $chan 1 1 /ckUsers $chan 
} 

alias ckUsers { 
  if $nick($1,0) < %minUsers { part $chan Channel Does Not meet Required Users Of %minUsers  } 
} 

on *:QUIT:#:{ If ($nick($chan,0) < %minUsers) { part $chan Channel Does Not meet Required Users Of %minUsers   } } 


Note: I reccomend You Leave This At 3.