Add contact to same company

Sorry, you can't do that. Address Book is designed for one contact person per card, with four email addresses allowed. If you want more contacts than that for the same company, you have to set them up as separate contact cards.

Here is one work around.

Create a card with all the constant information. Drag it to the Desktop. Now, still in Address Book, fill in the rest of the information for your first contact. Click Edit to exit Edit mode. Drag the partial vcard from the Desktop and drop it into Address Book. It will create a new contact with the same company information. Fill in your second contact. Repeat as needed.

I have the same complaint with Address Book, plus one more (no easy/fast way to enter multiple people at the same company/address, kludgy way of getting both Name and Company to print on address labels). For me, the answer ended up being a combination of Apple's Numbers and Avery's Design Pro for Mac. Apparently they're both limited in what they can do, but plenty powerful for what I need. You can try out Numbers for free as a 30 day trial (the iWorks bundle), and Avery's program is free. Once the cell headers are properly named in Numbers, you can simply drag info from Address Book, then edit or add as needed. It is ridiculously easy to add multiple people at the same address, and also very easy to mail merge this info into Avery's label program, by exporting the Numbers data as CSV. I've never used Excel, but I had this all figured out in a matter of a few minutes. If you need some pointers, let me know.
Dan