Yes. Multiple mobile devices (users) can be logged into the same Gateway cloud account simultaneously using the same email login and password.
Additional mobile devices can simultaneously log into the same account provided you use the same email and password. You will not need to add the sensors, nor the gateway to these additional mobile devices devices. This is because the sensor(s) and gateway(s) are already associated with the account.
Just download the app to an additional mobile device. Open the app to the main screen (the screen with the word "Devices" at the top), and then select the settings button in the far upper left. Once in the settings screen you will see the option to login.
After you log in using the same login credentials, turn off the Bluetooth switch below in the "Configuration" section. Then return to the main screen of the app where you will see the sensor(s) and gateway(s) on that account. It can take several minutes to download all the data from the cloud.