All core maintainers can work on these branches, however the noted maintainer for each branch is often the first to contact and the most involved in that particular area.
For the current stable release we have 7.x-3.x as the main branch for all the core projects. This follows the Drupal 7 default.
Feature branches can best be prefixed with 'feature/' resulting in 'feature/[issue number]-some-change'. Pushing them to GitHub and creating a PR for it lets us test the results with Travis.
These feature branches don't have to be pushed to D.o, and can be removed after being merged.
When a PR is not ready you can prefix the title with: [WIP].