Is the employer allowed to pre-register his employees in the system as part of the invitation, whereupon a corresponding invitation e-mail is sent out?

This is to be regarded as covered by the employer's right to issue instructions, but in any case must be justified by the employer's legitimate interest under Art. 6 para. 1 lit. f) DSGVO. Since the employee has a right of objection in the latter case and is informed about the data processing, all requirements for this are fulfilled. If the employee decides to see the newsletter as part of corporate communication, there would not even be a right of objection, as the employee would then not be able to assert any higher interests of his own. This is also comparable with the internal use of cloud systems such as Office365, SharePoint or Outlook, with whose operators contracts for data processing must also be concluded and where comparable business data of employees (name & e-mail address) are "pre-registered".