So let me get this straight.
- Each community is essentially a room.
- 100,000 can join, but only 8 can race at a given time.
- Rules (Kart speed and item selection) are set from the get-go and can't be changed.
- There are going to be several Nintendo-created communities, in addition to user-created ones.
- Between popular communities and the ones you're part of, you can select from a wide list of communities to play depending on your mood.
So essentially, this is like Zynga Poker on Facebook. You pick a room/community based on rules, and if it's not full, you start playing.