One email receives all communication? e.g. separate users sending invoices, but one receiving all responses. How to setup separately?

I'm happy to see you here, @rachael7.

You'll need to make sure that the email which receives all the responses of your sales transactions is not set as a Blind Copy (Bcc) on your setting. This can be the reason why only one email gets these details.

To do these:

1. Click in the Gear icon at the top.
2. Select on Accounts & Settings under Your Company.
3. Hit Sales.
4. Refer to the Messages section.
5. On the Bcc box verify the email that is set in that section. You may also double-check the email address on the CC and Email me a copy at section if its correct.

I'm adding here an some links that you can check out about setting up an email address for sales transactions in QBO for your reference: 

• Email and customise sales form
• Email a sales form or report to multiple email addresses

Reach out to me here if there' anything else you need. It'd be my pleasure to help you out. Wishing you and your business continued success!