While you could probably display the plugin’s login form in a popup (modal), it wouldn’t necessarily be a fully javascript solution since the default form would trigger a page load on posting the form (although I have seen users do it with the default form).
But because the plugin relies on WP’s framework and db schema to manage users, you aren’t locked into using just the plugin’s login form. Any login that WP recognizes, WP-Members would recognize.
I do have an article with code and explanation of how to implement a jQuery modal. This is on the plugin’s support site and is available to premium support subscribers. I don’t generally post links from the support site unless they are available for the general public; but in this case, since this isn’t directly part of the plugin at all, I’ll provide the link for anyone who is interested. This and other articles like it are available to support subscribers.
https://rocketgeek.com/tips-and-tricks/create-a-login-modal/
