Your alliance consists of everyone on your friend list and anyone you play with. You get extra alliance xp for playing with friends. Each week, you get a number of coins based on your alliance level and l think the top 5 xp earners in your alliance. So, it’s a social thing that pays you coins without you having to do anything active.
Actual guilds will be coming in the second update of Act 1, so probably Late March or April.