Only Outlook.com group members or trusted senders can email a group. The trusted senders list allows select non-members to send email to a group. A trusted sender is a person or email address whose messages you want to be delivered to your group. Without marking an email address as a trusted sender, the messages from that email address won't be received by your group.
Add a trusted sender to your group
At the top of the group card, select the Trusted senders tab or select the Trusted senders link under Group email address.
Select Add sender.
Enter any email address.
You can add or delete senders from this list at any time.