Click on Actions and then click on Collaboration . Under the Who Has Access section, there should be a list of users you've shared the presentation with.
When collaborating with someone for the first time, their email will be marked as "pending". This requires the user to click the link in the email that was sent to them to view the presentation. This authenticates them as a collaborator, and moving forward when you share a presentation with them, it will automatically show up in their library.