I’m currently playing in the NA East Sasha server. My friends are all spread out in different servers due to the various closed/reopened server status during launch day and the following days.
Since matchmaking is possible across different servers, we should able to add friends to our friend list on different servers. Usernames seem to be unique to the region, not to each server, so there should be no duplicate usernames.
By utilizing our region wide friend list, we could easily join “post-main story” events like raids and dungeons with friends from different servers with ease. Right now, the party request system makes it extremely hard for people to find each other.