Alliance – gives an ability to send assistance and make Joint Attacks.
Alliance existence is checked only at the moment of mission launch.

In order to form alliance you need to:

Alliance considered formed right after the offer acceptation.
Alliance rupture takes 24 hours. Alliance rupture cannot be cancelled.
You can allow your ally to see your domains or do not, as well as missions.
A player can have up to 5 allies.
Maximal number of allies at the moment depends on the "Diplomacy" science level.