On the context menu of the note,

Add Author to List

Display popup of existing list names and allow defining a new list.

One place to display lists is the top bar (where New Threads, Conversations is), horizontal scrolling to access/view the lists. Short press to view. Long press to edit members of list, and options for rename or delete.

When viewing a user profile, can add another tab denoting the lists they are on, with checkbox to add/remove.

Maybe the New Threads, Conversations and Global views need a menu choice to affect the view to toggle a list (for filter exclusive of or inclusive). But yeah, UI can get messy, and want to avoid many levels deep to effect a change.