I think only a single user per site can receive fax IN THE APP

If you want multiple users to receive fax you can have them receive the fax PER EMAIL.

Here is an example:
Ext. 125 is fax recipient.
Click on Users > ext. 125 > Notifications > messaging notifications > switch to Advanced settings > Customize notifications for Received faxes
Check the box to include attachment with the email. 
Add multiple email addresses to receive the notification

See 2 pics below

Noteworthy is that RingCentral has “Co-recipients” for voicemail, but this is not available for fax. 

Co-recipients means that a voicemail message can be received in the apps of multiple extensions. If you check out the ideas portal, you will find a feature request about expanding this functionality to fax. You could leave a vote on it.
 

I think only a single user per site can receive fax IN THE APP

If you want multiple users to receive fax you can have them receive the fax PER EMAIL.

Here is an example:
Ext. 125 is fax recipient.
Click on Users > ext. 125 > Notifications > messaging notifications > switch to Advanced settings > Customize notifications for Received faxes
Check the box to include attachment with the email. 
Add multiple email addresses to receive the notification

See 2 pics below

 

 

I don’t have the option to include attachment with email 

I’m wondering if it’s because we have turned on the HIPAA setting