The thesis must demonstrate the author's ability to apply relevant scientific methods and to provide research work equivalent to international standards for PhD degrees within the academic area, see PhD Order § 11.
A PhD thesis is an independent work, based on an individual PhD Programme, and therefore cannot be submitted by multiple authors for assessment. The underlying research project, however, can indeed have been carried out in collaboration with others.
Special rules apply for PhD students under the Industrial PhD scheme in relation to the submission of the PhD thesis and procedures for the appointment of the assessment committee, etc. Read more in the guide to the Industrial PhD Programme page on the PhD Order website.