Here're three scenarios in which you would be unable to add a specific co-author to your submission:
Scenario 1: If an abstract is assigned to a reviewer, then this reviewer can not be added as co-author in the same abstract.
Scenario 2: If organization check is enabled, then the reviewer of the same organization as one of the co-authors of the abstract, can not be added as a co-author.
Scenario 3: If the co-author has already been included in maximum number of submissions that is allowed by the organizers, submitter would not be able to that co-author on additional abstracts.
