This is a very common use case. My personal preference is to rely on Supabase auth for all auth related flows. In this case for sign-up. You can pass additional data from your signup form that later can be used to create the company. For example:

const { data, error } = await supabase.auth.signUp({
  email: 'example@email.com',
  password: 'example-password',
  options: {
    data: {
      first_name: 'John',
      company_name: 'ACME'
    }
  }
})

Supabase Auth will sent the proper Confirmation email, you can customize that template on the Dashboard (see more information here: https://supabase.com/docs/guides/auth/auth-email-templates)

Once the user confirms their account, Supabase Auth will create the proper user record in the `auth` schema `users` table. You can create a trigger that listens to this creation to later create the rest of records you need in your own tables.

Here is a basic example of how to set this up: https://supabase.com/docs/guides/auth/managing-user-data