Ok so mechanic and negotiate are kind of obvious; but what about infiltrate or the one I really want to know about counterspelling. Can you use teamwork on those skills.

I'm pretty sure counterspelling is explicitly allowed. I think it says that if someone is getting counterspelling from multiple sources to treat it as a teamwork test.
Infiltrate? Ummm, maybe. I wouldn't let a team of four guys all get teamwork bonuses for each other and collectively disappear, but if you have someone distracting people for you, or maybe someone running overwatch (matrix or astral) and giving you a heads-up then maybe teamwork tests would be a good way to handle it.

Yes, you can teamwork Counterspelling, in fact it says so in SR4 in the Counterspelling section. As for teamworking infiltrate, I would allow it depending on how the infiltration was taking place. For example, if one character were going to be making a distraction during which the rest of the team were to infiltrate the grounds of a facility, hits generated by the distracting character's test would apply, in my games, to the infiltrating characters' tests to represent the degree by which the guards were distracted. As always, YMMV.

In other words, all skills could probably be teamworked in some way. Does it make sense for two people to teamwork a pistol skill? Not in the sense of two people pulling the same trigger, but I imaging a time where a character, we'll call him Lando, is helping another character, perhaps named Han Solo, aim "just a little higher..."

Edit: and before anyone questions me (if in fact, someone were about to do so) I do allow teamworking with different skills, in my game. The above Star Wars example would most likely be a perception test by Lando (heavily, negatively modified if by nothing else the distraction of getting pulled into the Sarlacc) whose hits add to Han's heavily modified (at least by glare if not by straight blind fire) pistol's skill.

Teamwork for Infiltrate: Queer Eye for the Obvious Guy...

When you say "Counterspelling," do you mean for spell defense or for dispelling?

For the former, turn to page176 in your hymnal.

For the latter, beats me. I'd say no, because of the drain thing; that seems more like ritual dispelling, which sounds odd.

For infiltration, I'd also say no. I'd allow one character create a diversion for another character, which would then become a penalty to the observer's roll.

Don't forget: you should be naked and covered in orange paint and using a bullhorn (sorry, tutori (I think it was you), but it's a meme now!)

While you're at it, broadcast a ten-meter-tall AR image of yourself for that extra -3.

Ok, so how about conjuring? (i know that ritual spell casting is kind of a teamwork test for spellcasting, but what about spell casting?)
How about electronic warfare, and hacking?
How about piloting? esp. when there's a rigger jumped into the vehicle?
I know what I'd do. Apply the common sense rule, but it's be nice to have a little more guidance or even a list of skills (in the skill chapter perhaps) that listed what skillls could use teamwork and which couldn't.