The answer here are based on our discussion.
I assume that you have this specified this in your web.config
as enablePasswordRetrieval="false"
from your comment
Then this code will work for you
try
{
object guid = Guid.NewGuid();
MembershipCreateStatus status;
MemberShipUser member = Membership.CreateUser(register.Username, register.Password, "an email address here", null, null, true, guid, out status);
}
catch(MembershipCreateUserException ex)
{
//handle exceptions
}