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Most courts will give a schedule for discovery and disclosure at some point throughout the process. When a trial date is set, the judge should give each party a deadline for submitting a Trial Brief. Among other things, this document lists every witness that you believe you might call at trial. If the other party served you with discovery requests, read them carefully to see if they ask anything about potential witnesses. There's an obligation to update discovery within a reasonable time when you receive new information, so that could be another reason you might need to tell them. If they have not served you with any requests for information about your witnesses and the judge has not ordered you to disclose it, you don't have to tell them.
How opposing counsel is notified depends on why you're telling them. If it's in response to a discovery request you'd send a Supplemental Response (which looks like the original response, but you only answer what you're supplementing). If the court ordered trial briefs, then you'd file that with the court and mail a copy to the attorney.