To add an author to an article, you must have the article open in the Editor. You can edit the properties of the article in the left column. Among other things, there is the Author field.
With a click on Add you have the possibility to add one or more authors. If you want to select more than one person, you have to click Add several times and add one person at a time to the list.
When you are done with the selection of the authors you can save the article with the Save button and publish it with a click on Publish.