There is no option to hide the Online Members Widget for guest. You can hide it using custom CSS but the widget should be located in wpForo Forum Sidebar. The CSS will not work if you put the widget in WordPress theme sidebars.

https://wpforo.com/wp-content/uploads/2020/05/wpForo-Sidebar.png

This css code should be inserted in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea, then save it, delete all caches:

#wpforo-wrap.wpf-guest #wpf-widget-online-users.wpforo-widget-wrap{
     display:none;
}

I'd like to hide members online widget to the guests. Is there a way to do that?

Thank you,

Alice