Does anyone here know anything about, or have any interesting ideas about societies e.g. The Harpers, Red Wizards of Thay, the Zhentarim , that the Forgottem Realms had in them before NWN. Hmm new sentence 'cause that one was getting unbearable. Does anyone know if these societies will be joinable by PCs? It would not be hard for a DM to create NPCs which belonged to one, for a certain scenario or campaign, but how about Vault(TM?) Characters joining these societies?

Any interesting ideas?

This has been debated at ALFA. My understanding is: These organisations will exist, they will be run by the DMs, they will not be available for the players to join, at least at low level. Think about it, why would (eg) the Harpers be interested in a 1st level 'peasant' like you?
As you grow in experience and reputation they may want to 'use' you or 'set you up', depending on your alignment. Later they may approach you more openly.
Lots of possibilities for 'intrigue' which will make NWN a lot of fun!

The Harpers use a network of people and agents, some of which are peasents, same with the Zhentarium.

But its good that these massive groups are not available to join, they would cause too many problems.

that'sm cool but what about the ordinary guilds how do we find out about them
-Snorri

YOu role-play around talking to other characher and finding out information. Just think of it in the same way you found out about stuff in BG or in any PnP campaign.