Hey Brynn I'll try and lay this out for you.
The Bioware site contains a resource for guilds and allows them to have their own forums and such.
Neverwinterconnections is a very similar service, except that it also allows you to see games that other people are running and have open to the public.
When playing multiplayer you are basically entering a world on your own (unless you are playing an established game like one on Neverwinterconnections with a group). There is no set leader to a party. Other players can invite you to a party, or you can invite them to a party and team up to explore a server. There are alot of servers out there ranging from roleplaying oreinted to hack and slash. Some "worlds" are application only, while others require hak packs to play. Most if not all these haks can be found on the worlds website, or they are offered for download at places like NWVault.com and similar sites.
I would reccomend registering with Neverwinter Connections as you are new to the multiplayer community, it offers alot of opputunities to find a game or even a server that has what you are looking for.
Hope that helps if you have any other questions feel free to send me a message:
shadedragyn@yahoo.com
Shade