Office 365 Microsoft articles.

Delete or restore user mailboxes in Exchange Online

https://technet.microsoft.com/en-us/library/dn186233(v=exchg.150).aspx

Configure Office 365 Groups with on-premises Exchange hybrid

 https://technet.microsoft.com/library/mt668829(v=exchg.150).aspx

http://robsgroupsblog.com/blog/how-to-write-back-an-office-group-in-azure-active-directory-to-a-mail-enabled-security-group-in-an-on-premises-active-directory

CLOSER LOOK AD ADCONNECT

http://blog.enowsoftware.com/solutions-engine/a-closer-look-at-azure-ad-connect-%E2%80%93-part-5