If your test group is made up of staff members on the site, they should be able to receive emails sent from the site. Discourse notifications do not automatically generate emails though. There are a couple of things you can check.
First, make sure that the users are receiving notifications for the group messages. When you create a Discourse group, that group’s default Notification level is set to “Watching.” This means that users will receive a notification for every new message in a PM. This default setting can be overridden in the group’s Interaction section. The setting can also be overridden by individual users. If a user changes their notification level for a group inbox to something other than Watching, it will affect their notifications:
If your users are receiving notifications on Discourse, but are not receiving emails, the most likely cause is that they are active on the site at the time the notification was created. If a user is on the site when the receive a notification, by default, Discourse will not send them an email about it. Users can override this behaviour on their preferences page:
Users can also enable Mailing list mode. When enabled, they will receive a notification for every post created on the site.