restrict wordpress multisite subscribers access to content on sites they aren’t assigned to

Solution:

You can check if a user is a member of a site with this method:

https://codex.wordpress.org/Function_Reference/is_user_member_of_blog

So your code could look something like this:

$portal_left_link = get_field( 'portal_left_link' ); // the link for the marketing page
if (is_user_logged_in() && is_user_member_of_blog()) // check if the user is logged in and a member of the blog
{
    $left_link = $portal_left_link;
    $left_link_text = "Go To Marketing Page";
}
else
{
    $register_global = get_field( 'register_login_link', 'option' ); // trying to find this value
    $register_override = get_field( 'register_url_override' ); // trying to find this value
    $register_login_link = ( ! empty( $register_override ) ) ? $register_override : $register_global; // not sure yet what this does
    $register_login_link .= '?redirect_to=' . $portal_left_link; // concatenates a redirect for the (button) link to the portal page to be carried over to the register/sign-in page for when the user needs signs-in
    $left_link = $register_login_link;
    $left_link_text = "Login";
}
// code for button
<a href="<?php echo esc_url( $left_link ); ?>" class="btn"><?php echo $left_link_text; ?></a>