When you are adding the app to a device for the first time, during the set-up process, you will be asked 'Would you like the user of this device to be able to track other devices you might add to this account?' This is where you choose the users level of 'Account Access'
There are basically two levels, Full Access and Basic Access, if you select YES at this stage you will be giving the user of the device FULL access to the account; they will be able to view the other devices that you have added and any tracking information, and also be able to add and delete devices, change device names... basically anything that as the account holder you can do.
If you would like to limit the access to your account for the user of the device you are adding, then simply select the NO option. This will mean that the user will have the Basic Access; the app will track in the background on the device, and the the user will be only be able to use the locate now tool.