For more on quests, read our Quests page.
Are there group quests available?
Yes, most definitely.

Does each member of a group receive a quest reward if they participate in completing a quest?
If each member of the group accepts the quest from the quest-giver and completes the quest, then they also receive the quest reward.

What happens when a player fails to complete a quest? Is there an option to try that quest again?
Yes, players can always go back to the quest giver and try again.

Are there quests that take you to enemy towns and/or cities? (For example, can a Rogue sneak into town, steal stuff, and sneak out as a quest?)
Yes, there are quests in the game that take you into enemy towns.

Are there very large, or continuous, quests that require many steps to fully complete?
Yes. There are small multipart quests, which you might be able to finish in several days, and epic quests that span your entire adventuring career.