First of all you need to become a Vampire. Since you're asking this question, I'm going to assume you already did this. So, what you need to do is quite simple to join a clan. You need to go to a place where there are Vampires (Caves, Ancestral Tombs, etc.). Then you need to talk to all the Vampires until you find one that can give you quests. You can usually pick these characters out of the bundles of Vampires that are in some places because they always look really important.
The Categories of this question is so wrong.
Factions you can join: Companions College of Winterhold Dark Brotherhood Stormcloaks Imperials With the Dawnguard DLC: Volkihar Vampires Dawnguard
You cant. It's your only opportunity to get her.
You don't unlock her. Once you reach a certain point she will join with the team.
The Dark Brotherhood Sanctuary is located in the Cheydinhal East Gate.
The Dark Brotherhood is simply a side-quest in the game. It isn't crucial to the storyline but being in the Dark Brotherhood allows you to get weapons/spells/etc. that are normally unattainable.
You don't "join," you are asked.
I think that only happens if you have the 'Tribunal' expansion or are playing the 'game of the year' edition which includes the expansion. You can stop the assassins from coming if you travel to Mournhold and follow the quest there. for the Dark Brotherhood Assassin to attack you you must first reach level 6 or 7 cant remember which. Then you simply sleep and a message will say you have been awakened by a loud noise, and BAM. An assassin will be trying to kill you.
You are unable to join them, but you can wear their armour. :)
In original, non-edited Morrowind, the best Light Amour is Glass, the best Medium Amour is Ordinator and the best Heavy Amour is Daedric. in game of the year best light is glass or Dark brotherhood, best medium is adamantium or royal guard, and best heavy is deadric or high ordinator, especially the Her armor, which is high ordinator that increases all of your stats.
in a way yes rouge did join the brotherhood but later on she did come back to the x-men.
You don't need any skills to join, but for you thievery it is useful to work on acrobatics, security and sneak.
There are no others you can acess other than the 1 in cheydinhal