To add a custom restriction message for content that is restricted by Group or Member Type, use the
rcpbp_restricted_message filter located in includes/restricted-content.php.
An example can be found here.
$message - string, restricted content message. $is_group_restricted - boolean, shows if this content has an unmet group restriction. $is_member_type_restricted - boolean, shows if this content has an unmet member type restriction.