User management needs to be done on your server end. Please refer to the Web SDK guide to understand how to manage/sync your users.
Going forth, you will have to directly use the user’s ID to login the user. You should be able to get the user’s ID from your database.
You will have to use our Restful API to manage users.
Once your user has been created, you will need to keep the ID handy so that you can use it to login the user.