An audience can be created and only specified a set of contacts.
To create the audience, click on Your Account on the top menu and then on Settings.
The click on Audiences on the left menu.
Then click on the New Audience green button. In the new screen enter:
Name: the audience name.
Entity Type: what type of entities the audience will contain.
Include all contacts: indicates to include all contacts in the audience. Only tick this if you're looking to create an audience for all contacts.
Description: a description of the audience.
Configuring the contacts
To configure the contacts click on Add Existing Contacts.
Select the contacts and then click on Save.