First of all, you need to validate your e-mail address by activating the link in the mail you received just after signup. Once you've done that you can tap the + button top right in the Users screen (tap Menu-Users). From there enter the e-mail address of the new user and then tap either invite as inhabitant or invite as helper.
Inhabitants get control over the alarm like you have - but you may restrict their rights a bit after they have an active account. You do this by tapping the user name, then tapping the contact menu (...) top right on the user info screen. From there you may restrict the user rights by marking the user as a child.
Helpers don't get control over the alarm, but will be called in to assist an alarm that no inhabitants can handle. Provided that "call in helpers" have been activated under estate settings. This means that the alarm propagates to helpers after inhabitants to help you get a response even though you family is away on holiday.
PS If you did not receive a user validation e-mail, please look in your junk mail folders for it, and if it's not there then contact support to sort this out.