Champion Quests are pretty close to the only thing to do in Darkness Rising, so they're tied in with most everything else. They're also the only significant part of the expansion that can be done pre-50.

Shepherding does for CLXP what it does for MLXP - provide a reason for those who have done the encounters before to help others with them.

To receive shepherding experience, you have to:

• Have completed the Champion Quests.
• Be grouped with a player who will receive the large CoF/boss experience bonus (someone who hasn't killed that mob before and is on the CoF or final fight step).
• Be within 2000 units.

You won't receive nearly as much experience as the player doing the encounters for the first time, but you will get half a level if you shepherd someone through the entire process. The experience bonus will not change depending on how many in the group need the encounter (assuming there's at least one, anyway).

Albion Chapter One Walkthrough
Albion Chapter Two Walkthrough