Each user must have their own app to track their own results. If you want to share your device with multiple people, here's how you can do so:
If you have the Aim device (it's black in color), then you can store up to 6 users on a single device. You can add a new user by selecting "Add New User" through the Settings menu on your device. Each user will need their own app. Be sure to select the correct user before taking measurements, so the measurements will be saved to the correct user's app.
The Aim also has a Guest Mode which you can select on the device and measure a guest without storing their results to your app.
If you have the Chisel device (it's white in color), then it can only be connected to one user's app at a time. However, you can re-pair it to a different app at any time and as often as you'd like. Be sure to pair it with the correct user's app before use.