The table lists all your users.
If you are administrator of the current account, you will see all the users that have access to at least one environment of the account. If you are not administrator of the current account, you will only see the users that have an explicit access to the current environment.
To manage a given user, you can open its detailed view by clicking on it.
Click on Add a User to add a new user in any role for your account as described in the Add a User chapter.