It sounds like GW2 will have auto partying but here is how I see it. Let's say you leaves town by yourself to enter the battlefield. You see many allies and many foes engaging in combat. Partying with EVERYONE you see is impossible and in the chaos it is impossible to pick up party members. (No one is going to care about your "Awesome Warrior LFG" when everyone is fighting for their lives. ) So what do you do?
This may seem very chaotic but it is actually very simple for a soldier on the battlefield: YOU HELP ALLIES CLOSEST TO YOU AND YOU FIGHT FOES WHO ARE CLOSEST TO YOU. That was exactly how they did it back in the Roman and Medieval times and perhaps Anet should adopt it.
What I suggest is an auto ally pick up system.
1) You are automatically allied with the closest 20 allies nearby. The closer the ally the higher the priority. (You are near these guys so you should help these guys)
2) Allies who are outside of the "in the area" range and not "locked" are auto kicked from the party. (This guy is running too far away so you don't care about him anymore)
3) Players have the option to "lock" allies if they choose. (If players happen to find good allies they wish to stuck with they are allowed to do so).
This system will allow anyone to jump into large scale combats anytime, without the need to pick up allies in the town or having to gather at a specific time.
During the combat, players will be allied and un-allied with other players constantly. This makes sense since you only care are those allies around you. You can't really care about your father if he was fighting a mile away from you. That was how it was in ancient real life combat too.