1. Click on the Organization you'd like to add a user to
2. On the next page, you'll be on the Organization's Dashboard. Click on Users at the top right.
3. You'll be able to manage all Organization users here. To add a new user, click on "Create New User"
4. Input the Organization Users information and adjust their permissions (Notifications, Password views)
FYI: If you ever need to quickly manage an Organization User you can also "Disable, Delete and Enable" their logins.