Each user needs to have their own Balance Health 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 (A/B).
You can use the same device if you wish, you would just have to go to Account 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 being on the incorrect user.