Nothing happens when I click to add a member from a Group, Check-in, or Contributions screen.

Set your browser to allow for pop-ups.

From the Group, Check-in, and Contributions screen, adding a new member prompts a pop-up window for adding the person.  If you do not see this pop-up box, it is likely that your browser's pop-up blocker is on and blocking Churchteams. 

Oftentimes your browser itself will give you the option to choose to be an allowed site.  In other browsers, you may have to add as an allowed site for your pop-up blocker through the browser settings.  A quick search on how to "allow pop-ups" for your browser should help you fix this problem.