Listening on direct messages for a list of users



Currently I am researching a way to create a platform to listen for incoming direct messages for a given user and respond through an application to the sender of the message. I want to be able to listen to direct messages for a list of users and be able to respond to their messages through my application. I’m not quite sure where to start on this, upon initial inspection I would think it would be in the Site Streams, tracking users who have approved my app, however this feature appears to be in closed beta with no public access announced in the future.

Is there a way to achieve this outside of the site streaming api?