As an Administrator, you can allow submitters to add co-authors by searching from the existing users or even add a new/missing user to a submission. For detailed steps on how to enable Co-Author Collaboration, go here
From within a submission, you can add/remove Co-authors, and primary authors and re-arrange them as needed. Under the Actions section, click Revise/Edit
Add a co-author
Click Add Author, and search the user by name or email address. Click Done, and then Save at the bottom.
Remove a Co-author
Click the X button next to the author to delete, then click Save.
Assign/unassign Primary Author
Check/uncheck the Primary Author checkbox next to each author's name. Note, you can have as many Primary Authors as desired. Click Save once ready.
You can drag and drop authors in a desired order. Click Save to complete the changes.
Manually Added Users by the Submitter
When allowed, the submitter can add a new/missing user when initially submitting. These users are required to confirm their participation, in the meantime, you can confirm their pending status with a yellow frame around their name and the Manually Added button that will display their name, email address, and comments from the submitter about this user.
As an Administrator, you cannot override the pending status. For these users, however, you can still manage them with any of the above actions.