As of today (July 2016) the mailbox limit for a shared mailbox storage is 50GB (Not 10GB). To access a shared mailbox, a user must have an Exchange Online license. Shared mailboxes don’t require a separate license. However, if you want to enable In-Place Archive or put an In-Place Hold or a Litigation Hold on a shared mailbox, you must assign an Exchange Online Plan 1 with Exchange Online Archiving or Exchange Online Plan 2 license to the mailbox. Source: https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx