add custom field in user register form

To be able to do both – i.e. add something to the registration form AND have it editable from the admin side, you need to do 2 things.

The first, is add extra user meta (so you can see them from the admin side) and the second is to create a new registration template which hooks into WP’s registration process.

There’s a great tutorial at:

http://webcache.googleusercontent.com/search?q=cache:MO4DlL7hGQkJ:www.cozmoslabs.com/1012-wordpress-user-registration-template-and-custom-user-profile-fields/+&cd=1&hl=en&ct=clnk&gl=au&client=firefox-a

Which covers pretty much everything. Good luck!