Availability: Business and Enterprise.
Enable the feature in the Add-on store.
you can only send a document on behalf of users who have access/can view the document.
"Send as" functionality is available when you're ready to send out the document. So once you've finished adding content and setting up your document, click "Send".
Modify the document name if needed and proceed. In next popup, click the dropdown arrow in the "from" field and choose a person within your team (it will show only those with appropriate permissions).
Notification emails for "Send As"
Even if you're sending a document on someone's behalf, you will still receive all notification emails about this document.
Additionally, you can set up notifications so that you'll receive all notification emails about a document sent on your behalf.
Go to Settings in the bottom left > Profile > scroll down to Notifications > Check “Any messages or notifications for documents sent on my behalf”.