VS 2010 - Having Email Address As Unique Identifier?

I'm creating a website with sql server db (built in membership database). I design it best possible. Im reluctant to use text fields as identifiers, but this is pretty much my first sql server db.

1. The user registers himself, at the same time registering his company/organisation. I dont think many of them will have the need to register themselves to other companys (N:N) but you never know (anyway not a main concern at the moment).

2. This user becomes a superuser (level1), with the ability to register other users to the same company (level2 users). Finally level1 can register level3 users to the same company.

So you see the identifier will be everywhere in the sql commands. There are quite a few N:N relations as well, which I guess slows it down even more. The selections typically retrieves a few hundred to a few thousand posts, but like I said, many will be in N:N relations.