To create a user with limited access, create a new user group under "Setup > System Administration > User Groups" and set everything so the group is read-only. Afterwards you can add a new user under "Setup > System Administration > User Accounts", there you can set the Primary Group to the newly created User Group. Please also have a look at the following site:
To allow access to certain devices, just open the sensor/device/group settings and set the access rights in the bottom of the settings.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.