Alliance system in Anthem is a kind of reward system, you’ll earn Alliance XP in the game when you play an expedition (which includes missions, contracts, freeplay, and strongholds). Here it is not only the points that you have earned counts, but also those that your friends have worked out.
Even the points of the players in your friends list that are not with you are among them. These XP will bring you rewards. However, according to the current status, only the points of the top 5 players are included in the Alliance XP Pool.
At the end of each week you will receive the game currency Coin. The number of coins you receive depends on how many Alliance points you earn during the week. Each player in the "Alliance" receives the currency. The Coins then you can use to buy cosmetic items for your Javelin.
The more you play with friends, the more points you’ll earn. This will be displayed in Tiers. As you reach higher levels you’ll be able to farm more coins at the end of the week. However, you cannot collect unlimited XP here. There is a limit that Bioware has not yet revealed.
Bioware would like to reward the interaction with friends. As the friends also contribute to the Alliance XP pool, which is not currently in a mission with you, it also creates an incentive to make friends with as many of your fellow players as possible.