When I saw that you expected users to join with their GitHub accounts, I assumed that the exercises would be made available on GitHub and the submissions would also happen through GitHub (on a user specified repo).
You can find the exercises on GitHub (see https://github.com/exercism/xhaskell for example). They have their own workflow for the review process ("nitpicks" and "iterations") which could be grafted onto GitHub issues and pull requests, but it's not necessary and might just make beginners more self conscious about their stuff being open to anyone on GitHub.