on *:START:{ set %minUsers 3 } 
on 1:INVITE:#:{ 
  if ($me isop $chan) { halt }  
  else {
    timer $+ $chan 1 10 /ckUsers $chan 
  }
}  
alias ckUsers { 
  if $nick($1,0) < %minUsers { part $1 You need %minUsers users for me to stay. } 
}