signup.rb

class Signup
  include Virtus

  extend ActiveModel::Naming
  include ActiveModel::Conversion
  include ActiveModel::Validations

  attr_reader :user
  attr_reader :company

  attribute :name, String
  attribute :company_name, String
  attribute :email, String

  validates :email, presence: true
  # … more validations …
  
  include User::Validations # add User model validation

  # Forms are never themselves persisted
  def persisted?
    false
  end

  def save
    if valid?
      persist!
      true
    else
      false
    end
  end

private

  def persist!
    @company = Company.create!(name: company_name)
    @user = @company.users.create!(name: name, email: email)
  end
end

user.rb

class User < ActiveRecord::Base
  
  # you can extract this into a app/models/user/validation.rb but I prefer to have
  # little pieces of function like this in one place
  module Validations
    extend ActiveSupport::Concern
    included do
      # add validations here
    end
  end
  
  include Validations
  
  # ...
  
end