Currently there is an Action 'Send email from a Shared mailbox', however, it does not work for O365 groups. I have to use 'Send As' in regular send an email action.
It would be to great to add the functionality in Action 'Send email from a Shared mailbox', that it can also send FROM an O365 group mailbox.
We now released a new connector for Office 365 Groups mails (conversations). Unfortunately, Group conversations, while similar to emails, are different. You can find the connector here: https://docs.microsoft.com/en-us/connectors/office365groupsmail/
This is a huge issue since MS decided to remove the ability to create a Site Mailbox. Having a Site Mailbox made this easy. This is definitely a major issue for us. Either make triggering Flows, and sending emails from an O365 Group easier, or bring back Site Mailboxes!
We can use 'From' but ensure that Office 365 group has group delegation (find and click the Office 365 group > group delegation> add send as/onbehalf) with appropriate users added. This can only be done through Exchange Admin.
The @smerchatn solution resolve my problem and now I'm able to send emails from our groups.