Err...

If I remember correctly, in one of the latest letters from the devs, it was mentioned that the DF would be revamped on HWs release to admit incomplete parties into dungeons, for example duos or trios for 4-man content, or even entering Coil with a 7-man party. Later on allowing others to enter your party in progress.

This is one of the features I am personally looking forward to from HW.