You are the crowd in this context, and Guild Wars 2 uses events to collect groups of players and channel them between theme park rides. In the early days, when everyone is the same level, this creates massive zergs that are fun although not necessarily optimal. This will really pay off in the coming years as a solution to the common problem of finding groups after the population center is at the level cap.
First, we collect the players. You do this by creating an incentive to visit a spot and keeping that incentive there for several minutes. This funnels players together from whatever they are doing in the area. Have you noticed how every third town has an event where waves of enemies attack it? That is what is going on there. Everyone in the area gets an alert to the event, so they rush to town to defend. The incoming content is just short of continuous, so your natural impulse is to stay, keep fighting, and finish. Four to seven waves and/or one big boss is enough time for people to accumulate.
The tutorials have a more blatant example: keep fighting minions until the door to the boss opens. Humans fight centaurs at the bridge and asura fight robots at the gate. This lets players pile up in waves and face the boss together. This is a quiet tutorial in “work together with whoever is around.”
Second, we channel the players. Now that you have a group, you keep it together. Those big defense events are immediately followed by another event. Win or lose, you chase the enemy back to its base, either to press your advantage or to recover whatever the enemy took. Your pickup group moves together following the NPC, trail of crumbs, or just the crowd. Who needs forced grouping when the game makes it the natural choice? Who even needs the “group” mechanic half the time?
As I said, this is neat and/or silly now, when you have an entire legion of charr descending upon the centaur camp or smashing the Flame Legion. Fast forward a few years. Think about how empty the low-level areas seem in most games, especially in off hours. GW2 will have the same population density shift, but the event design will naturally bring people together. You will not be stuck soloing because grouping is inconvenient or suboptimal. You do not even need to want to group; the content brings you together, and you can address or ignore these people as you please.