Well, a lot of people seem to be unable to find friends in shops...have they stepped into the twilight zone?
What happens is that when a shop is full (a certain amount of players) the game automatically 'clones' the shop. So while you may be in 'Zone: 1" your friend may enter the shop and be in "Zone: 4".
To know which one you're in do shift+F9, it will display more details than when you just press F9.
The best way is to meet right in front of the shop outside. The same applies to dungeons, however the only one which really is full almost around the clock is Temple of Three Winds
in Greater Tir County. If you are teamed you can always enter the same 'zone', however it is best to meet outside totw before teaming.
We'll be sharing more tips about the game in the coming weeks as we are working on updating a few guides.