I just released user-inbox 0.8.0. It is ready for CUBA Platform version 7.2 and furthermore allows sending messages to multiple receivers. Check it out at https://github.com/mariodavid/cuba-component-user-inbox.
Also, there is a new example, based on CUBA petclinic that uses the user-inbox addon: https://github.com/mariodavid/cuba-example-using-user-inbox
Here are some demo usages from the example:
Manually Sending to multiple receivers
Using Sharing feature
Programmatic Creation of Messages