A User that logs into the system can be set in the system as a restricted User to only view the Calendar, keeping your Data safe. To do this you would first create a Sales Person for that User by following,

Menu > Setup > Sales People > New Sales Person.

Fill in the relevant fields needed and click Save.

Next we would create the Sales Person a User Profile and set them as a restricted User by following, Menu > Setup > Users > User Profiles.

Under Defaults and Filters you can then select the Sales Person you created - this would then prevent the User from seeing any Appointments other than the ones set for their default Sales Person.