Pick a username, enter your primary email address, and pick a password. Your username will be publicly visible, but your email address is never revealed to anyone.
Click "Create an account".
When it shows the "Choose your plan" screen, click "Sign up for one of our free plans".
When it shows the "Tailor your experience" screen, you can either tick whatever boxes appeal to you, or click "Skip this step" to continue with default settings.
Click the green "New issue" button in the top right.
Create a new issue with a title like: "Please add me to the Github organisation!"
Click "Create new issue".
In the next few days, an administrator will send you an invite by email. You’ll just need to click the link in the email, and then you’ll be a member of the DoESLiverpool Github organisation, able to edit and assign people to issues in the "Somebody Should" issue list.