Adding email addresses to your email client is easy with Thunderbird.  First, open your Thunderbird client and at the top of the screen you will see a box that says "Address Book":

Once that is selected, a new window will pop up that shows any addresses already in your Address Book.  In this example the Address Book is empty.  We will click on "New Contact" to add a contact to our Address Book:

On the next screen is where we will fill in any information we want about the contact.  You can fill in not only their email address, but also their phone number, address, and even a picture.  Once you have filled in everything you want to add (you can add more later if you need to), click on "OK" at the bottom of the window:

Now your new address is added!

You can repeat this process as many times as necessary to add all the contacts that you need to to your address book.