JOSS, the Journal of Open Source Software - if you’re making good quality research software, you deserve credit for it! You can publish relatively short papers for your open source research software, and/or review software, too.
http://www.firsttimersonly.com/ try your own first contributions, then recruit others for your projects!
Hacktoberfest: https://hacktoberfest.digitalocean.com/ (Octobertime!) - get stickers and a t-shirt if you complete 4 pull requests for an open source project.
Student? Learn how to work open source by contributing to an Open Source project for three months over the summer (students only): https://summerofcode.withgoogle.com/ (Applications in March-April)
Do you lead an open project? Apply to participate as a mentor organisation, or join an umbrella organisation. (Applications in January)
More on GitHub
Syncing forks
GitHub docs: Merging an upstream repository into your fork
StackOverflow: How to synchronize fork with original GitHub project?