Double click on the list or enter in the list and click on New Contact

I want to add a whole new name and contact to the mailing list. It does not give me the option to add a contact. I don't want a new contact (it keeps giving me options for a new list - I do not want that...

Add a contact to Personal Address Book or Colleted Address Book, in mailing list just write the email of that contact and the Thunderbird will recognize all the information related with that contact

I am still not able to figure out how to add an existing contact in collected email group to an existing group. Please advise.

The easiest way is to drag and drop the contact from the address book to the list.

Open your address book window. Do you have the folder pane showing? If not select View-layout Folder Pane

Now all your address books and any lists that you have created in them will display as folders in that pane. If the lists do not show, click the small pointer beside the address book folder to expand the view. Click the folder for the address book so it displays in the main window. Find the contact. Click the contact and drag them to the list folder and drop them there.

You can also double click the list name to open the list edit dialog box and just type in the name of the contact.