Set it to
$New_EmailMessage/To +','+$IteratorEmployee/Email
and behind the loop you will need to remove the last comma-character from the string. You can do so by changing $New_EmailMessage/To like this:
substring($New_EmailMessage/To, 0, length($New_EmailMessage/To) - 1)
Besides the correct answer from Tim van Steenbergen above, please consider not committing the New_EmailMessage
object in a loop (or any object as general rule).