The capability to grant Send-as and Send-on-behalf permissions to an Office 365 Group is a straight forward process and clearly documented here.
Sending an email on behalf of an Office 365 Group via Outlook is the same familiar process as sending on behalf of another user – show the “From” field, click the “From” drop down, choose “other email address” and select the address.
What is less intuitive is doing this using the Office 365 web interface via portal.office.com
First you need to show the “From” field by clicking the arrow below the compose page and selecting “Show From”
When you click on the “From” drop down, you will not see an option to choose an alternative address
Instead, what you need to do is right click on the address in the “From” field and choose “Remove”
Once removed, you can type the address you want to send as