Adding new users can be done from your "System management". Click on the settings button on the upper right corner, and click on "Users" under "People & Organization". 

Here you can see the list of users you that have been added to the system. To add a new user, just click on "+" button and choose "Add a New User". 

You can now see all the possible settings that can be changed for this user. Fill in all the required fields, set up permissions for this user and click "Add".
The user has now been added to the user list.

If you want to have your users divided in different groups, you can also create these very easily. Go to "User groups", click "+" button, choose a name for your group and you are done!

Now, if you want to add a new user or edit some information for someone already added, you can check the groups you want this user to be added like this:

That's it!