You could add your address to the “TO” line and enter the rest of them in the BCC field. None can see each others.

If you wish to create a group (category):
This worked for me, but I’m in Yahoo Classic.
First you must have all contacts in your address book.
1. From your Yahoo Mail page, click the Contacts tab.
2. Check the boxes next to all of the names you want in this category.
3. Click the “Add To Category” tab above the address book.
3. In the new drop down menu, click on “New Category.” 4. Type a title for the group/category in the box that opens in the upper left. Click OK.
4. When you wish to send an email to the group, click on “TO” to open the address book.
5. Click on the blue “categories” link in the top of your mailing list, for mailing.
6. When your list of categories opens, check the box next to the “title” of your group, go to the top and click on “Insert Checked Contacts.”