Yes. However, ecobee devices can only be registered to one ecobee account. You can login to the ecobee account from multiple mobile devices using the same username and password. For example, you may wish to share thermostat access with a family member by allowing them to access the ecobee app through their mobile device.
Simply download the ecobee app on any additional devices you wish to use and login to the app using the same registered username and password.