Easiest way: Add/change a file via Githubs interface, send a PR.
Each file in _post is one meetup event. See the example in _drafts to see which fields will be handled (drafts will not be rendered).
Get the correct Jekyll version with gem install github-pages (requires Ruby 2).
Then, start a server with jekyll server -w --baseUrl="". It rebuilds your site when you change files.
You can enable drafts of posts by adding --drafts.