Not sure what you mean...Do you mean that all the coins collected by all the players go into a community pool to unlock new things? That has been confirmed, but I can't remember where...
In previous games where you could play the GP mode with multiple players, you could unlock characters and such exactly as normal.
Worst case scenario : only the player associated with the NNID the game is played on gets its coins added towards the total count, in which it's the same thing as playing the game alone anyway. You shouldn't be worried too much about this.
Basically, 4P GP from the beginning even if it doesn't unlock stuff makes this a co-op game. Without it, it's mostly a VS game (although I was happy to see the Team options in MKWii). Being able to unlock stuff is icing on the cake, but I much prefer to unlock stuff with a co-op feeling than from single player only. MKDS was the last time I was excited enough about a MK game to play it a ton single player; MKWii quickly became multiplayer only (I'm not sure if I even bothered unlocking every Grand Prix for 150cc) and I completely skipped MK7. I am only finding out now that track design and kart differences/viability had started to get better when they added gliders and underwater sections. a number of the tracks I've seen in MK8 now impress me with route options that make even off-road tires useful at times, anti-grav collision boosting that makes weight even more of a consideration, and track design that reminds me more of Crash Team Racing than any previous Mario Kart. So I'm excited, but if I can't immediately sit down with friends, play GP together, and unlock stuff all at once I'll end up a bit tempered with my excitement. It's not a deal breaker on whether or not I'll have fun with the game, but more of an issue of how much trudge I'll have to deal with solo in addition to the multiplayer fun.
I'm generally a co-op maniac and even refuse to play Tales games alone.
Yeah I was talking about unlocking grand prix and 100cc/150cc/Mirror modes.. maybe even stamps