Fixes $wpdb->prepare() logic in bp_blogs_record_existing_blogs()

The parentheses were in the wrong place.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6929 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Don't query for entire user objects in bp_blogs_record_existing_blogs()

When recording existing blogs, we only need user ids of blog members. So it's
only necessary to query for the ID field when using get_users(). This should
result in significant performance enhancements on large installations.

Fixes #4856

Props borkweb

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6930 cdf35c40-ae34-48e0-9cc9-0c9da1808c22