The introductory Guildhests are a must.

Basic Training: Enemy Parties (Level 10)
Under the Armor (Level 10)

Both the above are very fast and should cover roles in a reasonably non-lethal way.

Basic Training: Enemy Strongholds (Level 15)
Hero on the Halfshell (Level 15)

Reinforce on what the two starter ones did and get the point over that you must pay attention to the objectives - especially in Hero on the Halfshell. How many of us have screamed at people to quit attacking the turtle!

All these are fast to complete and also not a bad way to get a bit of extra coin and experience as you are questing.

I encourage people to do them all before the story quest dungeons.