The corresponding author requests to add an additional author before publication

Clarify the reason for the change of authorship
Check that all authors agree to add the additional author

If all authors agree, then:

Obtain a declaration of authorship from the new author (if used)
Correct contributor details (the role of each contributor or author), if published
Proceed with review and publication

If the authors do not agree, then:

Suspend the review or publication of the article until authorship has been agreed upon by all authors, through institutional management if necessary.

Note: Important changes to respond to reviewer comments, e.g., adding new data, may justify the inclusion of a new author