Can we add co author later?
In principle, it is possible to make changes up until the point that final proofs have been submitted. However, at a well-run journal any attempt to add an author after acceptance would require approval from the journal's editor, and they would likely require an explanation for why this change is being sought before agreeing.
The reason for this is to guard against various forms of misconduct, especially:

subversion of the review process, e.g. by concealing conflicts of interest;
offers of authorship as a reward/bribe.

If the editor is satisfied with your explanation, they will allow you to add authors. If they are not, the result is likely to be that the paper's acceptance is rescinded, and it is rejected from the journal. Note that many journals require the corresponding author to certify that they have included all relevant individuals as authors as part of the submission process, and admitting that you lied here does not do much for your credibility.