You can register the not-listed-users in setting the global access.
They can be a members of the website in clicking "Website information"→global access of "Control the access" to check "Anybody can be the member if they are the users with valid log-in ID".

How to participate:

In clicking "My work space"→"Membership"→" The website you can be a member", the website will be displayed. Please click "Be a member" then.