Each user needs to have their own Weight Gurus account, so you'll first need to set up a new account for the second person using a separate email address. After that, you'll just go through the same setup process as the first user, just being sure to choose a different user number (U1-U8).
You can use the same device if you wish, you would just have to go to App Settings in the app menu to Logout and then Login to the other person's account. It is easier to each have the app downloaded on your own devices if that's an option. Save you some time on logging in and out all the time and also save you from accidentally weighing on each other's accounts.