Hi Demi Vesinger,
Thank you for sharing on Microsoft Community. We are happy to help you here.
When you create an Outlook Private Group, you tenant will automatically create a Microsoft 365 group. By default, the option for "Let people outside your organization email this team" is turn off. Therefore, when external user (outside your organization) emails this group, the email will bounce back.
To turn on this feature, please follow these steps: Go to Exchange Admin Center > Teams and groups > Active teams and groups > Search for this group in Teams and Microsoft 365 groups > Select this group > Settings > tick on "Let people outside the organization email this team > Save. It will take couple of times to take effect.

Note: If you are an end-user, please contact your IT admin to do this for you.
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Greatly Appreciate again for your patience and understanding.
I wish you keep safe and wish you all the best in year 2025 as well!
Sincerely,
Nam-D - MSFT | Microsoft Community Support Specialist.