For your users that are configured to only have access to selected vehicle groups :
Here are the steps to edit the "Allowed vehicle groups" :
Click this "Allowed vehicle groups" menu:
Click this "Edit" button:
Then select the groups you want this user to have access to:
This way, the user will have access to nothing else than the data from the vehicles that belongs to the selected groups.
Alternatively, you can also assign vehicles to groups while editing or creating vehicles: