This Office 365 Help page from GoDaddy should address your problem.
Send as a distribution group address
If you are an Office 365 admin, you can give members of a distribution group the permission to send as the distribution group address.
- In your Office 365 account, click on Dashboard.
- Click Launch Office 365.
- Log in using your Office 365 email address, and password.
- Go to the Exchange Admin Center.
Only the primary admin account will be able to log in to the Exchange
Admin Center.
- Under recipients, click on groups.
- Select the distribution group you want to edit.
- Click Edit.
- Click group delegation
- Decide whether you want to assign Send As, or Send on Behalf permissions.
Send As
This allows a member of the group to send email as the distribution group address. To a recipient, the From line will display the group, only.
Send on Behalf
This allows a member of the group to send email on behalf of the group. To a recipient, the From line will display the sender "on behalf of" the group.
- Click Add, and find the members to whom you want to assign Send As and/or Send on Behalf permissions.
- Click Save.
In order to send as the distribution group, you need to show the From field in your email composer. To show the From field in Outlook, open a new email, click Options, and select From.