Prerequisites: Main Building Level 1
The maximum number of people in an alliance is equal to 3 times the level of the highest level embassy within that alliance. Therefore with a level 20 embassy up to 60 players can be in an alliance.
The level dependent attributes (such as construction costs, generated culture points and maximum alliance members) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.