Read the GitHub guide to forking projects and making pull requests.
If someone has made a pull request and you would like to add code to their branch, there are a number of ways to move forward. It will be very helpful to get familiar with managing remotes in Git.
First, ping them in the #admin-ui channel on Drupal Slack to discuss the addition/changes Once agreed, you can make a pull request to their fork/branch, which once accepted will appear in the pull request to this repo
Alternatively, they could give you access to modify their branch directly at https://github.com//admin_ui/settings/collaboration (although bear in mind this will give access to their entire repository fork)
If you're unable to agree, or unable to get in touch with the author, you can create a new pull request that incorporates the work from their branch. Leave a comment in the old pull request linking to the new one.