Put the Piece interface and all created pieces directly into the MASTER branch.
The MASTER branch is available to everyone in the repository.
For every other class, when commiting(saving) the file, send it to your own branch, preferably use your name to name it so its organized.
That way, others can't edit it and multiple people don't work on the same file at the same time.
For example, I need Elliot's Board.java file, I can send a pull request to the "Elliot" branch. From there, I can "edit" his file but not directly change it (Google Doc commenting). Then Elliot can view my changes and manage it from there.
Once everyone finds an agreement to the file, commit it to the MASTER branch so it is accessible to everyone.
*** NOTE: editing a MASTER file at the same time as someone else will cause errors in code.