it was a drag when I played the beta, since there was no way of knowing what 'adventures' people in the group had, so it was like this...
Leader: Let's rescue the princess in "The forbidden cavern!"
Wizard: Hmm... don't think I have that
Leader: Oh, ok, then let's defend the poor villagers at the "Fort of Sunshine"!
Rogue: Sorry, I didn't buy that one
Leader: hmm... so does anybody not have "The singing hobgoblin"?
(silence)
Leader: Great! let's all go there.
(everybody gets inside except one, 30 seconds later...)
Cleric: Sorry, was AFK. Oh, no I don't have this one.
The good thing is that leaders can set a message in the "looking for members" interface (which is awesome, and something all MMOs should have) saying they're going for a specific adventure pack and it's paid, but you'll still get people who don't bother to read the text.