To add a new problem to the repo, no solution needs to be present. Just add a folder to the problems directory with a readme detailing the complete problem and expected result.
To add a new solution to the repo, add the solution to the correct language directory in the solutions directory. Make the solution file name match the problem. If a solution already exists, but your solution is distinctly different to the current solution, place both solutions in a directory named after the problem. The file names should then accurately represent the type of solution used in each solution. If you must use multiple files for your solution, create a solution directory and another directory for your solution assets.
In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality.
Please lint and test your code with any means available - currently JavaScript has tests and linting via Mocha and JSHint.