Will MettleCI have access to our company's private data?

MettleCI will not have access to your company’s private data, unless you explicitly provide it, in which case your Developers could commit it to your Git repository.

MettleCI deals almost exclusively with data management artefacts (jobs, sequences, parameters, etc.). The only component of MettleCI through which customer data passes is the Unit Test Harness when used in Interception mode. That captures your supplied test data into test data files which are then (potentially, depending upon your settings) committed into your application’s Git repository and propagated to downstream QA environments as part of your jobs' automated Unit Tests.

In summary, MettleCI does not expect or require access to company data, but any data that are provided may be shipped to any downstream environments which feature in your deployment configuration.