BuddyPress.org

Try this plugin:
https://wordpress.org/extend/plugins/restricted-site-access/

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Just place this between opening and closing php tags in the bp-custom.php file:

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On the below line you can change the /register to the page you want them redirected too!
wp_redirect( get_option(‘siteurl’) . ‘/register’ );
For example:
wp_redirect( get_option(‘siteurl’) . ‘/activity’ );