My employees received an invite to tsheets though I never signed up or authorized such an email.

It's possible that you accidentally created a TSheets account, sweetcw.

You can let your employees ignore the invitation if it's not valid. However, we can verify if you have a TSheets account by signing in to this link: https://www.tsheets.com/. Then, enter your possible logging in credentials. 

If you're able to log in, you can cancel the subscription. Just go to Company Settings and then Account & Billing. Select Cancel Subscription. A confirmation email will be sent once the cancelation is complete.

In QuickBooks Online, you can keep track of the activities your users made in your account using the Audit Log report.

Let me know if you have other questions. Take care!

I verified I do not have a TSheets account. I reviewed the audit log (good suggestion), and confirmed there is no mention of TSheets in the audit log.