How to add a domain and users to a current account

OK, so the shared mbx eliminates the need for licenses, correct?

So in my scenario I have the current "company1.com".  I add "company2.com".  

I have 2 users in both company1 and company2; bill@company1.com, melinda@company1.com, bill@company2.com, melinda@company2.com.  I then create a shared mailbox called everybody@company2.com.  I make Bill@company1.com and Melinda@company1.com members.

In the mbx scenario bill and melinda each have their own, private, email in company1.  But they can each see the others email, both sent and received in company2, correct?  If I (the customer) sends email to Bill@company2.com, how does that get routed into the mbx everybody@company2.com?  Is it because I have made bill@company1.com a member of the mbx everybody@company2.com?  Will Bill have manually select the From address each time he replies or will the reply come from everybody@company2.com?  I don't understand how it would know that email for Bill@company2.com belongs to Bill@company1.com

This may not be an issue, I need to check with the customer.  In fact it may be an advantage.  But if not then I will need to purchase (or allocate) 2 licenses, one for Bill and one for Melinda, correct?  I wish there was a way to test this before hand, I do NOT want an email disaster on my plate.

Sorry if I'm being too **bleep** about this, but I would like to understand it.