Unlike other games you can be in five guilds at once AND see chat in all five at once. You can get into the banks on any of them or their guild stores on any toon you make. Once invited all characters made will be automatically in the guild due to it being account based.
Due to the alliance system, if you don't make an Ebonhart Pact character, you will miss out on the main family stuff. All our AvA events will be Ebonhart based. Fog will tell us which campaign our Ebonhart Pact toons will be fighting in. You will want to put any character in the other alliances in a different campaign if you want to AvA with them outside of doing Ebonhart with the family. This way you won't wind up out there facing family on the wrong end of a sword. =)
Briseadh will be a Nord Templar as my main, but I will probably play with a Kajiit and might keep them in their traditional alliance for a change of pace. Then again they might wind up EP, too. I have an idea for a black Kajiit named Kata'Strophy that would work as a cross over to Ebonhart. ;)