In OWA, as happens with Outlook, you can add any shared MailBox where you have permissions:
Navigate to your OWA in Office 365 and select the Inbox folder in your Mailbox so a contextual menu is shown. From this menu you can add a shared Mailbox by clicking on the Add shared folder… option.
In the modal dialog that opens, search for the Office 365 Group you want to add and then press the Add button:
Then, you will see the Office 365 Group added to your OWA folder structure. Of course, you will be able to access to any e-mail sent to the Office 365 Group and also send e-mails from it if you are granted to.