If I'm understanding your question correctly, you want to know when submitting an article to a journal who can be listed as an author?. Also, is that limited to co-investigators? Some journals do specify who should be listed as authors but I have not seen that limited to co-investigators. For example, someone who reviews your article before submission who is not a co-investigator but makes a contribution to the article can be listed. Here are some general guidelines for appropriate author listings;

Someone who

• Contributed to some parts of the article and participated in interpreting the results.
• Was involved in writing a draft of the article or revising it for intellectual content
• Reviewed and approved the final version before submission. 

@Madeleine Lawrence yes that is exactly what I am asking about. At the time of submission itself, if I happen to need to add more authors than those just listed during the time of registration of the trial, provided they have made meaningful contributions to the manuscript.

Thank you