Like Nillen pointed out, you can add code to your on join section of the code. The way I would do this would be to have the bot write each new person's name to a txt file when they join. Have the bot check first to make sure the name isn't already on the list before adding. If the person's name isn't on the list, then give the welcome message AND add them to the list so that on their next join they won't be greeted again. Something like:
if (!$read(joins.txt,nw,$nick)) {
msg # Welcome to the chat, $nick $+ !
write joins.txt $nick
} 
