BCC- Blind Carbon Copy
BCC - Blind Carbon Copy.
If you want to send a message to more than one person, you can define the e-mail address(es) here.
Click on the arrow button to the right of the BCC: field and select the person(s) from the MM address book. Click on the button to the left of each addressee you want and finally click the SELECT button to enter them all. The e-mail addresses are placed in the BCC: field, separated by a semi-colon ' ; '
When you send the message, MM creates the file BCC.TXT (Blind Carbon Copy) in the folder
..\MM_Data\MM_Groups
If you plan to send messages more often to the same group of people, you may want to rename this file immediately to e.g. MyFriends.TXT -- and create an entry in the Address Book for that group. The next time you only have to select that record to send a message to every one in the list.
You can find complete details on creating a Group Text in the Address Book topic here... 
Note:
If you need to send the same message to more than one Yahoo group (by adding other groups to the BCC line), then simply remove the [Group Name] from the subject line, Yahoo will prefix the subject with the correct [Group Name]. This eliminates having more than one group name listed in the subject.