If you have access to a shared/delegate mailbox or Office 365 group, you can send emails as your shared mailbox or group.
Be sure you have been granted Send As permissions to your desired delegate/shared mailbox or Office 365 group
You are now able to send an email as your desired shared mailbox or Office 365 Group in the Outlook Web Application.
You are now able to send an email as your desired shared mailbox or Office 365 Group in Outlook Desktop App.
You are now able to send as your desired shared mailbox on the Outlook Desktop Application for Mac.