How do I issue custom programming challenges? Any of GitHub repositories you grant access to can be issued as a challenge. Set up a private GitHub repository, commit challenge instructions (README.MD works well) along with any code and other files you want the candidate to receive to master branch and issue the challenge using Zero In.
What happens when a candidate submits their solution? Once a candidate submits their solution (or the timebox expires, if set – see next question), Zero In opens a pull request in the repository containing the challenge, against a per-candidate "review" branch pointing to the exact commit used for issuing the challenge. That way, you can keep updating the master branch and altering the challenge, while still getting a review diff representing work done by each candidate.
Can one candidate see other's solutions? No – each candidate is provided with a dedicated Git repository containing only the challenge issued, and can not see other candidates branches or solutions.