It depends on the kind of email address your administrator gave you. If your invitation includes a link to an account setup page, yes, you'll need a Yahoo! ID to activate the address. This allows you to access your new email address with the Yahoo! Mail interface, plus any other mail program that supports POP (such as Outlook Express or Thunderbird). To activate your email address, you can use a Yahoo! ID you already have or create a new one.
If your invitation only includes directions for setting up POP, you'll access email only with a mail program that supports POP, such as Outlook, Eudora, or Thunderbird. You can manage and retrieve email messages without using Yahoo! or having a Yahoo! ID.
Note: Each Business Mail account with Yahoo! Mail access must be linked to a unique Yahoo! ID. If you already own an address in a particular domain and would like a second one, you'll need to create a new Yahoo! ID with which to access it.