Why won't it allow me to create more accounts that use the same email address?
If we allowed this it would be a very big security flaw; Generally, standard security practices mean that 1 email per account is a safer method. This means when you forget passwords or a private email is sent to you etc. no-one else has access to those details.
If you wish you may use 1 account to pay for multiple people's registrations and member's fees etc, just put the member id it is for in the appropriate field.