The One With Barry & Mindy's Wedding - S2-E24
Corrected entry: Monica and Richard both attend Barry and Mindy's wedding as guest, but they have never met either the bride or groom. So it doesn't make sense for them to be there, other than for a location for their breakup.
Correction: Rachel and Ross could have been allowed to bring friends? Maybe Richard and Barry know each other through their jobs, seeing as they are both in the medical profession.
Cybermoose