Also, the "join in " option when trying to group across overflows and main instance is still a little broken. A few of us struggled for some time with this last night.
It seems that the easiest way to get this to work is to figure out which member of the group that everyone in the group has an option to "join to." It seems to randomly let you join all, one, or none of the people in your party, and everyone may be in their own instance :|
Once you find each other...everyone will get the request, when it happens, to rejoin the main server at the same time. I recommend this as it simply makes it easier to stay together from then on. Of course, there is the issue with greater lag in the higher-population server. But, you get more people on, more group events. ....This might help when you make the unfortunate decision to venture into, say....the tomb in the level ~9 Charr area to find that one skill challenge (that, IN MY DEFENSE!) was there throughout the betas. Here, 4+ rather upset bosses will spawn, on top of each other, more or less, in a very cramped area.
I apologize for leading us to certain death at that moment (at least I fulfilled that promise!)
...especially when we eventually realized that ANet fixed the major balancing issue of this zone (the level 10 gold super evil bosses stacking in a tight level 9 area) by simply moving the skill challenge elsewhere (it's a commune challenge)