A positive antigen test does not need to be confirmed with a PCR test. Rapid antigen tests have low rates of false positives so a person who tests positive on a rapid antigen test almost certainly has COVID-19.
You should follow isolation guidance and notify your close contacts. In the near future, you will be able to alert your contacts of a positive at-home test via an anonymous link using MassNotify. Currently, this feature is only available for positive results from official PCR tests, where the link is provided by Massachusetts DPH