Go to the publication. In the upper right-hand corner of the page, you should see the publication name/logo next to your profile photo — click on it. The last item in the dropdown list will be Homepage and Settings — click on that. At the bottom of that page should be a section called People. Under that will be the list of editors and then Writers. In the Writers section, you can add someone as a contributor. It’s also where you would remove someone as a contributor.
I assume all the publications are set up the same. That is how I added writers for SnapShots.
Let me know if it worked.