Well, you have to determine how you are wanting them to be separated, and you may wish to explicitly state that on your web page or something like that...
We'll start with the add_multiple.php page...
And, let's say the $emails variable looks like this:
$emails = "john@doe.com,jane@doe.com,sam@whatever.com";
So, you'd explode it:
$splitemails = explode(",", $emails);
Now, it is in an array, so you can loop through the array, and add each individually:
foreach($splitemails AS $v){
if(mysql_query("INSERT INTO table (emails) VALUES('".trim($v)."')")){
echo "Email added";
} else {
echo "Email not added";
}
}
Or whatever. The trick is to get it into an array. We add the trim, just in case there are spaces before or after the commas. If you need help with this, post back with your code, and we can help you tweak.