The maximum member count on all levels of guilds has been changed to 20.
When you join a guild, it will enroll your entire account into the guild, using only 1 space of membership. This means that you are counted as one guild member, but will be in the guild on any character you log in on. This also means that the Guild Master will have access to his/her special options regardless of what character he/she is on.
All of your characters will be registered to a family. You can play with your friends and still receive your benefits regardless of the character you play on.