You can send a custom campaign to a specific set of subscribers (one or more) without sending to your entire subscriber list.
How to use:
When creating a campaign, at the top right of the campaign settings you will find a To: dropdown.
Clicking on this will show you options to choose from:
All Subscribers - All your subscribers in your account
Custom lists - Any lists you may have saved from previous campaigns
Specific People - Enter one or more email addresses to send this campaign to.
For Specific People, you can enter one or more email addresses (separated by commas if you are providing more then one)
You may also Save as subscriber list which allows you to name this list for future use. If you don't save, it will just be used for this single campaign.