try this:

on *:PART:#: {
  writeini wb.ini $chan $nick yes
}
on *:QUIT:#: {
  writeini wb.ini $chan $nick yes
}

on *:JOIN:#: {
  If ($readini(wb.ini,$chan,$nick) == yes ) {
    .notice $nick Welcome back....
  }
  Else {
    .notice $nick welcome to....
  }
}