Train or taxi into City?
We got the S-Bahn, as our hotel was right near an S-Bahn station.
Took 30 minutes. Was a piece of piss.
Their trains are MUCH more organised than here in the UK.
Regarding price, we bought the 'Berlin Welcome Card'.
Can't remember the price (about £20). You get unlimited train, underground, bus and tram travel for 3 days.
You then also get a discount on all the main "attractions", as well as restaurants, fast food, etc.
Not sure if you'll need it going in a big group, as you can probably already get discounts,
but would definitely recommend it for smaller groups.