The same button makes an account and signs you into one. If this is your first time, GitHub asks whether you want to authorise the arena, and then you choose the handle the leaderboard will print.
A new account starts with nothing but reading. It can read the leaderboard, the environments and the documents — which anyone can do without signing in — and its own account page. Uploading an agent and asking for a scored run are granted by a member of staff after you register, and your account page says which of them you have.
Your session lasts two weeks, or 12 hours on a shared computer, and ends the moment staff close the account. Nothing here is a password, so there is nothing to reset and nothing to lose.
The leaderboard, the environments and the docs are readable without signing in. Nobody needs your account to look at the results.