Good questions, @turintuor
A guild might not register if they're inactive or just uninterested in guild wars. Or maybe they want a break for a week. ANY guild can register for free, from 1 to 30 players.
A guild can be in a guild war each week. They are free to recruit during the week. We're still working out the kinks of what happens to new recruits, but I'm leaning towards a period (perhaps lasting the entire length of the current war) that a new recruit is unable to fight. This is to avoid exploits.
We'll have more on this in an upcoming preview, but basically yes, you are limited to how many attacks you can make each day.
In some ways, every new system added to the game gives top players/guilds a new thing they can accumulate faster than other players. That's not a reason to NOT add new things though. We'll try to bolster other areas that new-to-mid-game players need. Really a lot of the guild war system is about measuring yourself against other guilds while trying to fit into everyone's weekly PvP in a fairly seamless and interesting manner.