Search inside GitHub repos: click "All Filters" on the search panel, scroll down and you will see GitHub Repositories. Pick the name of GitHub project in the drop down. For example, if you want to source all contributors into Hugging Face' Transformers, start typing "huggingface/transformers" and you will see this project name in the drop down list, and click "Show Candidates" button.

Install Prog.AI Chrome extension. Go to GitHub, open the project you are interested in, click on "Contributors" to see all contributors (for example, here), and you will be able to see the list of people with contact info in the side bar (Prog.AI extension).