Do you mean how to add an author in ResearchGate? Then see the section "Adding, editing, and removing co-author information" in https://help.researchgate.net/hc/en-us/articles/14292798510993-Authorship for instructions.
Or do you mean how to add an author to the original article after publication in a journal? Then contact the editor(s) of the journal, asking for the publication of a correction to the original article.
If you need to make an authorship change, you will need to contact the Journal Editorial Office or Editorial team in the first instance. You will be asked to complete our Authorship Change request form; all authors (including those you are adding or removing) must sign this form.The title of the corrigendum must be "Authorship Correction: (old title)". In the corrigendum text to be published, describe which author has been added and at which position (after/before which author), and briefly the reason ("was inadvertently omitted"). Also mention whether or not the Acknowledgments or Conflicts of Interest section need to be changed due to the additional author, or how the addition affects any potential "equal contribution" footnotes.
The corrigendum needs to be submitted as new submission with all new authors in the correct order and with ORCIDs/affiliations as in the original submission, with the new authors added (or author removed).