When sending to multiple recipients, Employee Email will not be able to tell you for certain which specific contact has triggered each open. This is because when sending one email to multiple people (either in the To, CC, or BCC fields) they all receive the exact same email, which means there is only one tracking code used.
However, you can access some details about who opened an email, such as the location/device information that you get from looking at your sent and tracked email in the web app.
Employee Email was configured this way to work in line with the existing email workflow. For example, if a separate email was sent to each recipient it would make it difficult for them to use Reply all.