Self-invitation is not possible in public TDS.
When it's requested, it's possible to configure TDS portal to support self-invitation:
After entering proper e-mail address, invitation is sent to a new user. A new user is added after email confirmation to TDS portal only (not to Company/Project).
Once User receives to his email an invitation he can perform sign up and create his account.
User agreed, that he reads TDS Privacy Statement clicking button Sign me up.
Now he will be able to log in to TDS portal.