Board Thread:The Quarters - Q & A/@comment-108.167.6.51-20160218222833/@comment-24672610-20160219013656

Being in a party doesn't automatically mean you're participating in the party's quest--you have to also accept a quest invitation, and there's no way to join when the quest is already in progress.

You have two options in this situation:
 * The current participant(s) of the quest can finish it without you, and you wait and join them on the next quest.
 * The person who started the quest can abort the quest. In this case the scroll will be returned to her quest inventory, and she can start it again and wait for you to accept the invitation before restarting it. Any progress that has already been made toward defeating the boss will be lost if you do this.

You can read more on the Quests page.