Can AA's share their experience on the Big Book and the Steps it contains with members of other fellowships, e.g. Al-Anon, S-Anon, and guide them through the technical mechanics of the Steps?
Absolutely. Almost everything that applies to alcoholics applies to Al-Anons. Even the Step One is almost exactly the same. The alcoholic is powerless over the alcoholic's desire to drink and his subsequent excessive consumption. He can't control the course of his life. The Al-Anon is powerless over the alcoholic's desire to drink and his subsequence excessive consumption. If his life is tied to the alcoholic's, his life will be unmanageable too. They will need to understand alcoholism from the inside, and an alcoholic can be the best person to help them see that.
The added element with Al-Anons is this: being an Al-Anon means developing maladaptive coping mechanisms to deal with an alcoholic, essentially the four false beliefs of:
"I'm not responsible for me;
I am responsible for you;
You're not responsible for you;
You are responsible for me."
... and their many correlates and manifestations in belief, thought, and behaviour.
As an Al-Anon, I'm powerlessness & my life is unmanageable if there is an alcoholic on the rampage in my life, but I'm powerless over my own maladaptive response (and my life is consequently unmanageable), too.
So there are two levels.
If you're an AA, you will most likely also have that maladaptive response burgeoning in your present or past life, so there's totally a point of identification.
In any case, all problems solved by Twelve Step programmes boil down to this:
A persistent return to a destructive pattern. This is powerlessness plus its consequence: unmanageability.
The AA sponsor taking the Al-Anon through the Steps per the Big Book is not a one-stop shop, as in any other sponsorship deal. The Al-Anon should in any case study Al-Anon literature, go to Al-Anon meetings, and talk to a gazillion Al-Anons. Sponsors are just tour guides. Not masters of their sponsee's fate.