Changes in number of authors is not uncommon. There are many reasons for it, not by far only because of problems.
If you submit a manuscript to a journal and you need to have the authorship list changed (not just re-arranged), the easiest way is to wait until the reviews have come back and you, hopefully, are in a situation to make revisions. You then change the authorship list and provide the editor with a clear explanation of why the list has been changed.
It is of course common sense to send an e-mail to the editor as soon as possible and provide the information on changes and reasons why. The editor can then act as she or he sees fit. It could involve providing the reviewers with the information or not take any action at all, but at least you have been up-front with the information.
Remember that when you submit a manuscript there is commonly agreements in place that concerns all authors so providing the journal with changes in authors can be important to the journal.