How to Call Member Login via Pop-Up Upon Restricted Content Access Attempt

My use case for Simple Membership is pretty basic. On my portfolio site, I want to grant invite-only access to a select category of posts—no general registration, I distribute site links with usernames/passwords.

The category protection is doing exactly what I’d hoped. But the login experience feels overly complicated (too many redirects and clicks). First they’re directed to a message telling them to login or register, with links to login and registration pages. Then they have to click a link to go to the login page. Then they have to put in login details and Submit.

Is it possible to bypass a bunch of that? I want to trigger a pop-up window (containing login form short code) when someone tries to access a protected post. I’ve installed the Popup Builder plugin and can use shortcode just fine, but when someone clicks protected content, how do I bypass the interim “you need to login” message and call the login form pop-up instead?

Advice and insight appreciated.