From the GE Announcement You can find it in the Announcements forum.
Each guild member from Iron Age - will generate a cost of 2 Iron Age goods (or 4, when unlocking the difficulty III).
Each guild member from Early Middle Age - will generate a cost of 2 Iron Age goods (or 4, when unlocking the difficulty III).
Each guild member from High Middle Age - will generate a cost of 3 Early Middle Age goods (or 6, when unlocking the difficulty III).
Each guild member from Late Middle Age - will generate a cost of 4 High Middle Age goods (or 8, when unlocking the difficulty III).
Each guild member from Colonial Age - will generate a cost of 4 Late Middle Age goods (or 8, when unlocking the difficulty III).
Each guild member from Industrial Age - will generate a cost of 5 Colonial Age goods (or 10, when unlocking the difficulty III).
Each guild member from Progressive Era - will generate a cost of 5 Industrial Age goods (or 10, when unlocking the difficulty III).
Each guild member from Modern Era - will generate a cost of 5 Progressive Era goods (or 10, when unlocking the difficulty III).
Each guild member from Postmodern Era - will generate a cost of 5 Modern Era goods (or 10, when unlocking the difficulty III).
Each guild member from Contemporary Era - will generate a cost of 5 Postmodern Era goods (or 10, when unlocking the difficulty III).
Each guild member from Tomorrow - will generate a cost of 6 Contemporary Era goods (or 12, when unlocking the difficulty III).
Each guild member from The Future - will generate a cost of 6 Tomorrow goods (or 12, when unlocking the difficulty III).
Unlocking a difficulty makes it available to all guild members who take part in the current Expedition.