You can either:

• Click the To: or Cc: buttons on a Compose page to open your Contacts list, and click the name you want. To select more than one, hold down the Ctrl button on your keyboard while clicking the names. Or,
• You can just type a few letters of the person's name or email address, and Yahoo! Mail drops down a list of complete email address choices that match what you've typed. Click the one you want from the list, and Mail fills it in for you. To add another address, type a comma after the one you just entered, and then start typing the next address. Again, just click the one you want from the list, and Mail fills it in for you.

Sending to the same group again and again?
If you're sending to a large group repeatedly, you might want to create a category. That way, the next time you click To: or Cc:, you'll see the name of this group (category) right in your Contacts list, and you can just click it once.