is the co-op offline and online?
is it structured, like you have to have 2 or 4 people to progress?
can you save at a point and continue with different people?

think of offline co-op as Monster Hunter United (Monster Hunter Portable 2nd G) training mode
offline co-op mode can be single player or two players(split-screen)
think of online co-op as online gathering hall
one time only you get 20 day free trial
after the trial you need to buy wii points to play online

will people probably use psn/live to communicate with friends whilst playing this?

you can use wii keyboard(wireless or wired) to communicate with others or you can use virtual keyboard(not sure about this)
some people use skype