A Twitter-like app that takes seriously the social networking platform's essential purpose of allowing users to complain to large numbers of people.
First and foremost, use the issue tracker. It's the tab over to the side that says "Issues" ---> Find an issue that doesn't have an assignee, and assign yourself. If there's a job to be done, but that job isn't in the issue tracker, go ahead and add it. This will make sure that no two people accidentally work on the same thing independently.
After you complete your work, merge it with the appropriate branch, and then push it up. NEVER push directly to master. If you want to apply changes to master, merge your changes with master locally, push them up to a branch, and then create a pull request to merge with master. Be sure to let someone else accept your pull request; just accepting it yourself defeats the purspose of creating a pull request for people to review.