If an audience has been shared with you, you'll be able to view it by going into your Audiences menu, and checking your Current list (1):

You will also be able to see and choose the shared audience in your campaign or Quick Send Choose recipients module, just like any other audience.

To send only to the shared audience, tick Search for Contacts, Tags & Audiences in the Choose recipients module and start typing in the name of the shared audience. When it appears in the dropdown, click on it to select it.

If you select Send to All Contacts instead of choosing an audience, you will also see the Include Shared Audiences checkbox beneath it. Tick this if you would like to include all shared contacts in your campaign.

As a recipient of a shared audience, can I modify it?

If someone shares an audience with you, much like a list integrated from a CRM, you cannot change the audience list. You can however add or remove contact tags or unsubscribe a contact (affecting your individual user account only). Contacts cannot be deleted from a shared audience. 

* As Admins do not have contacts themselves, the ability to access shared audiences is only available at User level.