An email address is a very common way for modern websites to enable users to login to protected areas as it (usually) has the benefit of identifying the individual and it also provides key contact information.
There are a small number of instances where people, typically couples or parents and children, share a single email address and because the system cannot identfy which person is using the email address it won't let them login. There are two ways to overcome this:-
1) Each member uses their Membership Number to Login.
2) Obtain a unique email address for each member.
Fortunately this doesn't mean separate computers or even any cost, it is a case of choosing who you want to host your email address and where any emails associated with it are to go.
Free email addresses can normally be obtained via your Internet Broadband provider with most of them allowing up to 5 free email addresses to be created for each account. Signing in to your account will probably give you enough instruction to set a new one up if needed. Alternatively, a number of large organisations such as those below offer free email addresses, the only catch being that it needs to have a few emails pass through it each year.
Click on the link to set up an email address
Google - https://www.google.com/accounts/Login
Microsoft - https://login.live.com/login.srf
BT Yahoo https://login.yahoo.com/config/login_verify2?&.src=ym
You can access these email addresses directly via the internet or, if you use an email client (Windows Mail, Microsoft Outlook, Outlook Express, etc.) you will need to add this account to it so that you can send and receive emails.