You can be in several guilds even in the same character. You get to choose which one you wish to "represent" and earn Influence Points for at any given time.
I believe you get the same freedom across all characters...for example Character A being in Guilds X and W, Character B being in Guilds Y and Z.
I can't say for sure, but this is how I think it works. Honestly I haven't even touched the guild interface during the beta events. I will at launch, though. Plan on creating my lil' social guild for me and my buddies.
yeah if character A is in Guild X then character B is also in Guild X, but character B can choose to represent Guild Y instead of X. You can only see the guild chat and gain influence for the guild you are currently representing. Influence is also earned per world, so make sure you are on the world with the rest of your guild members when trying to earn influence. If your other guild mains on a different world, you can always "Guest" onto that guilds world as well.
This series is the Inception of games. Everyone is inside each other, and it only goes deeper... - DrunkBeardGuy on Kingdom Hearts Proud 3DS Ambassador