In the podcast from this post http://www.gamesas.com/topic/1428525-everything-you-need-to-know-about-eso-all-interviews/ I got the information that the player can belong to multiple guilds. So say you have a small group of friends you enjoy hanging with, you can form a guild with them, and you can also then belong to a larger Raid guild, and even a PvP guild if you want that as well.

What I don't understand is how you will manage the various guild chats, and I suppose you will have to switch chat channels and chat with each one seperately. They are apparently still working on that part but the idea of being able to belong to multiple guilds I think is fantastic!

Or possibly upon login you say I want to be with my PvP guild this session or I want to be with my friends guild, etc....and then relog to change guild chats...

I think it's good for guilds to have a really fluid role. They aren't a big gameplay thing in TESO, at least compared to something like Guild Wars, named for the very concept. You aren't tied down to one guild because the guild isn't the primary PvP element: that's more the role of the three Alliances, and TESO aims for a lot of TES players who are used to a single-player experience and will want a lot of solo content. So, instead of having PvP based around guilds, they can afford to make a far more dynamic guild concept, focused on a more casual social role as well as PvP. And it'll work really well with the Megaserver thing.