Board Thread:Questions and Answers Forum/@comment-98.17.202.245-20160222170410/@comment-25916964-20160222215151

If your in a Fireteam that is attempting to access content (such as a strike) that is part of a DLC that you don't own, the entire Fireteam will be unable to access it. Similarly, you'll be unable to join a Fireteam that is in an activity that is part of a DLC you don't own.

In the past, there has been an exception to this for some of the Crucible maps which were part of a new DLC. (probably for beta testing)

Oh, and technically you already have the DLC since Bungie adds each DLC to the Destiny game files as an update. When you 'download' the DLC, your essentially just downloading a key to that content.