This page shows an overview of all the users in each site. You can change the site you are viewing by changing the 'Site' dropdown. Once you are in the correct site you can add a new user by typing in an email address into the textbox and clicking 'Add User'.
Once a user is added you can select the pen icon to edit that users permissions. Which you can find out about more here: Permissions. You can remove a user by clicking the bin icon.
You can export the users to a csv by clicking the 'Export to CSV'.
You can import users by clicking the 'Import users' button. More information.
You can see all users in the study by clicking the 'All Users within Study' button. More Information