You address a mail memo to a group. When the group name you specify in the To: (or cc:) field is a public group name, located in the Public Name and Address Book (N&A Book), the group name does not expand when you send the memo (or press F9 to refresh the fields). You cannot determine, therefore, which individuals comprise the group.
If you place the same public group name in the bcc: field, the group does expand to show all the members. In addition, when you place a private group name (from your private N&A Book) in either the To or the cc: field, the group also expands.