Posting this here instead of the alliance forum because I didn’t want it to get drowned. I’ve seen this question posted all over the web but I couldn’t find any answers. So: at least as of this post date, being in an alliance does exactly TWO things for you:
1. Each time you get your daily reward, you get +10 iso for each member of your alliance who has been active “recently” (in the last day?).
2. Each time an event (PvP or PvE) ends that you participated in, your alliance gets ranked as well. Every member of your alliance (I believe) gets the reward for your alliance rank. (You may not even need to participate to get an alliance reward; I don’t know. Also I don’t know if alliances are banded like players or not, but I suspect not.)
None of these rewards are retroactive, and you must be a member at the exact moment those things happen to get a reward, although you can quit a few seconds later and it doesn’t matter. If anyone can clarify any of these points, please chime in.
So what does this mean for a new player? It means you should absolutely join any alliance you can find when starting to play (easy enough with the search function) and be sure you are in an alliance immediately before you receive your daily reward or before an event ends. Note that as a new player, you will probably get booted out almost immediately as well; I don’t know why people leave public, open alliances out there and then just boot anyone who joins, but they do. So try to join immediately before the event ends / you earn your daily reward. Also, the system is blatantly, heavily biased in favor of large alliances, so always try to join the biggest one you can find on the search list. 5 members are the default, but I think it goes up to 20 (maybe higher). Larger alliances mean higher daily rewards and better rankings.
Also, if you join an alliance and it has a terrible rank, feel free to quit and join another one. There is no penalty for leaving, and you’ll probably get booted anyway (I’m guessing most open alliances hope that a 3*, high-ranked player just stumbles in, as opposed to newbies). Try to find the best one you can as close to the event end time as possible. Or just bounce around to the largest one you can find for your daily reward.
The main thing to know is that there is zero downside to joining an alliance and you can only benefit. Lastly, alliances technically also have a chat function, but you’ll probably get booted before that makes a difference to you.