In the upper left-hand corner of the app on the home screen you will see an icon that looks like two people. Click on that icon to begin the call tree process.
You should now be on a screen that says, "My Team Members". In the lower right you will see a red oval that says "+ Invite". Click on this button and your phone will ask you permission to access contacts. Hit approve.
Here in your contact list choose the members you want to add to your call tree to respond to alarms.
On the "My Team Members" page you can hit the "edit" button and use the arrows to rearrange the order of the call tree members.