There are casual players, players who like to group, and players who liked to be involved in massive raids… can they all co-exist? They can definitely co-exist and even play together in an interdependent environment.
Vanguard's world will contain areas designed for casual, group, and raid experiences. Risk and reward will vary in these areas, as will the amount of playtime necessary to complete quests and other activities.
And these different areas aren't there just to appeal to people who always have more or less time. They are also there for the gamer who often does play longer hours but at times wants to log on and play a shorter session, or for the person with less time who wants to occasionally devote an entire Saturday to playing.
Source: Official FAQ