To login to Giles you need a GitHub account. If you do not have one yet, head over and create one. Once you have a GitHub account, click on the GitHub icon in the right upper corner, or on the button that says "Login with GitHub".
When you try logging in for the first time, an administrator will have to approve your account before you can start using Giles. Administrators should get informed automatically by Giles that you created an account, so will only have to wait for them to approve your account. After your account has been approved, you can login and start using Giles.