Code:
on *:JOIN:#: {
  var %user $nick
  if ($readini(rxcoin.ini,3michaelukz,%user,coins) != $null) {
    writeini -n rxcoin.ini #michaelukz $nick coins $calc($readini(rxcoin.ini,3michaelukz,%user,coins) + 1)
    var %var $readini(rxcoin.ini,3michaelukz,%user,coins)
    msg # Welcome back %user, you have earned another coin and now have %var coins. 
  }
  else {
    writeini -n rxcoin.ini #michaelukz $nick coins 1
    var %coin $readini(rxcoin.ini,3michaelukz,%user,coins)
    msg # Hello! And welcome you now have %var coin!
  }
}


Sorry, forgot that it would the coins would just be nulled. Hopefully this will work, although make sure you turn off the bot and then activate it again. Otherwise it won't act as joining.