It would be great if you could share some sample or dummy data in a .twbx file. I can give you a few calculation suggestions (below), but I have a history in web analytics and am aware that other elements might come into play; it would be helpful to have a sample of your data structure.
Users who Login //Note: the ELSE NULL is implied - you don't have to have it
COUNTD(IF [event_id] = 80 THEN [user_id] ELSE NULL END)
Users who Signup //Note: the ELSE NULL is implied - you don't have to have it
COUNTD(IF [event_id] = 81 THEN [user_id] END)
Users Not Signed Up
[Users who Login] - [Users who Signup] users_not_converted
Hope this helps!