To do this, first go to Manage-> Audiences.
On this page, you can see all of your Retargeting Audiences. (To learn how to create Retargeting Audiences, see here.)
Note the "Connect" column:
If an Audience's Connect setting is turned to On, you are sharing Audience with your partners. To share the Audience, click the "Off" button to turn sharing On:
(You can just as easily stop sharing an Audience by turning this setting to Off.)
We'll also let you know how many Partners are targeting your Audience. In this example, we have 13 Partners targeting our first Audience.
Once you've turned Connect on for a Audience, your Connect profile will update to show the Audiences as available for use.
You can update your Audience sharing settings at any time under Manage-> Audiences.
Once you and your Partners have shared Audiences, learn how to Target those single Audience.