You should make sure you included the catalyst documents when you emerged catalyst. It provides very well documented example spec files detailing this. Users are added with the livecd/users: section of the stage 2 spec file. The users that are created are non-root and if you want to add more than one, just seperate them with a space. The users added in this way are added to the users, wheel, audio, games, cdrom, and usb groups. The first user on this list will also be used to start X.