bp_members_add_role_after_activation( int $user_id )

function bp_members_add_role_after_activation( $user_id ) {
	// Get default role to add.
	$role = bp_get_option( 'default_role' );

	// Multisite.
	if ( is_multisite() && ! is_user_member_of_blog( $user_id, bp_get_root_blog_id() ) ) {
		add_user_to_blog( bp_get_root_blog_id(), $user_id, $role );

	// Single-site.
	} elseif ( ! is_multisite() ) {
		$member = get_userdata( $user_id );
		$member->set_role( $role );
	}
}