This is a huge problem in modern mmorpgs, where the game gives you systems that does all the work for you on cross servers. The people you team up with through the DF, they mean nothing to you and you mean nothing to them. It's not possible to add people you team up with through the DF to your friend list, and it is not possible to add people you team up with through the DF to a list consisting of people you don't want to group with or be matched with in the future.

The ability to make friends through play is imperative for an MMORPG to have longevity with players. This is especially true, because of the grindy and repetitive nature of MMORPGs. You need to have people around you to do stuff with. MMORPGs are inherently social games, because the content in the genre is very rudimentary.