You can join every guild, and you keep what each one teaches. An ability needs both your level in that guild and your player level to reach the level it asks for, and guild levels stop at one hundred.
Stat bonuses stack across guilds. Each guild raises its own two attributes by one every three levels, and each of the others by one every ten.
Spells have 20 ranks, a new one every 5 levels. Each is stronger and costs more spirit, and the stronger damage spells take longer to cast. Skills and class commands stay as they are, and grow with your attributes. Each guild's class command is the one thing only that guild can do.
Passives come from your people, not your guild. You will find every people's gifts on the Peoples page.