Articles are added by using blue colour Tab 'Add New' located on the top right of the profile page. Articles also get added by accepting suggestions given by RG.
However, you cannot add your own articles only, not of others.
If you are missing an article you co-authered you can add it. If not look in the profile of a few of the authors. The article might be in there publication list with a slight error in the title or another minor difference.
Article get added by their authors. If none of the authors is a ResearchGate member then the article will not be there.
You are better to look for articles with Google Scholar, and if there is a copy that is open access Google Scholar will tell you.
There is also an add-on for browsers called Unpaywall which looks for open access copies of papers found by Google Scholar. Both Google Scholar and Unpaywall include ResearchGate in their searches.