IMSS / Services / Email, Calendar & Messaging / Email Features / Delegate Access
Setting someone as a delegate on your mailbox allows this person to receive and respond to email messages and meeting requests and responses on your behalf. You can also grant the delegate additional permissions to read, create, or change items in your mailbox.
Instructions are provided by Microsoft in the link below.
If you are already set up as a delegate, below are links to instructions on how to open a shared mailbox or calendar.