Hi, welcome to Android Forums.
It depends on how you access your Yahoo account - using either the Yahoo Mail app or as an account added to your stock mail app, and also whether the LG Optimus Contacts app lets you create groups.
It would help if you posted again with that info or better still sign up as a member and have the benefit of a full dialogue. Just tap the link below.
Having said that you can send/forward to multiple addresses by simply adding the addresses one by one into the 'To' box - a bit tedious but it it does the job. Some mail apps automatically create a list as you enter each address (the HTC One does), if not then separate each address with a comma, colon or semi-colon.
If you are using the Yahoo Mail app you just go into your contacts and tick the appropriate boxes, tap 'Done' and the chosen addresses are entered into the 'To' box.
Hope this helps.