They only affect the alchemist unless he takes the Infusion discovery, afterwhich he can give the infusions to an ally, and the ally can drink them.
They got communal resist energy for synergy reasons. It provides no benefits above and beyond the normal spell for them, I think.
I have a rules question.
I've always thought that when an alchemist made an extract, it was basically like a limited use potion that could be only used by him (unless you take the infusion discovery of course). However, a fellow player interpreted the rules as you drinking the extract, then essentially "casting" the spell, aka an alchemist could drink a CLW extract, and then touch a fellow player to heal them of the damage healed from the spell. I was pretty sure that this was not the case.
Then, I saw that "communal resist energy" is an alchemist spell. What? I'm thinking that my friend must be right, as how else could this function? Either the alchemist essentially "casts" it when he drinks, allowing his fellow adventurers to also benefit from it, or he has to pass the extract around so everyone takes a swig. Besides the communal aspect, there is no difference between "communal" and standard "resist energy", so I'm really in the dark here.
The majority of alchemist extracts only effect the alchemist unless you pick up the correct discovery. Some extracts bypass this and say so in their descriptions.
Wands work the same way they work for everyone else. Alchemists don't use wands differently.
You're right. Your friend is wrong. The reason communal spells are on the alchemist's list is because the alchemist learns spells like a wizard, ie: by reading scrolls and books. It would be weird if the alchemist found a scroll of communal resist energy and couldn't learn from it; after all, its content is pretty much the same a regular scroll of resist energy. SKR confirmed this a while ago. It shouldn't be too hard to find the relevant thread. Do a forum search for "alchemist communal spells" or something.
Edit: Ninja'd!
Communal spells cast from wands work exactly like any other spell cast from a wand. They function at the caster level of the wand, and otherwise work exactly as if you had cast the spell yourself. Simply follow the spell description and you're good to go.
Energy type is chosen at the time of casting, so it's the wielder, not the caster, who chooses.