How to address a class about cheating
Please carefully read your school’s policies concerning academic dishonesty before you do anything.  Follow those rules carefully so that the students won’t be able to get away with their cheating on appeal.  For example, at my school you’re expected to talk directly with the student to give them a chance to defend themself before you decide on a punishment, and you’re required to submit an official report which allows the school to decide on any larger sanctions.

I’d also recommend having a third party (e.g. course coordinator, director of undergrad studies, or TA) present for the conversation.  This makes it clear that it’s serious, makes me feel safer, and prevents the student from lying about the meeting.