Register the signup table in the members global even if registration is disabled

Conditionally registering the table causes problems for our unit tests. It also
means that we have to worry about table creation when enabling registration
later on.

See #5374

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8123 cdf35c40-ae34-48e0-9cc9-0c9da1808c22