Lead is probably the best ability in the game IMO. It gives mucho bonus to defense and offense. This may sound crazy, but my access increases dramatically playing with a lead up (maybe an additional type ahead line?).
Anyway, the following should answer your questions:
1. You have to have a group and you have to be leading the group.
2. The duration is based on circle/charisma (I believe).
3. It is a pretty big drain on soul and you will need to replenish after a certain number of leads. Charisma determines the size of your soul pool (there may be other factors, but I only know Charisma).
4. As a point of reference, I can lead four times before I am drained with 35 charisma.
Good luck.
Madigan
True heroism is remarkably sober, very undramatic. It is not the urge to surpass all others at whatever cost, but the urge to serve others at whatever cost.
Arthur Ashe
