The One With Phoebe's Wedding - S10-E12
Answer: For one thing, it was a very quickly-executed affair, on the street outside the coffee shop with Joey officiating. Second, Phoebe doesn't have much in the way of family, and of those we saw over the course of the series: she and her sister hate each other; her grandmother passed away years before; she hadn't been in contact with her birth mother, nor her biological father, in some time; and her brother, the only one she'd likely invite, is busy trying to raise triplets.
Answer: Also, there was a pretty bad snow storm in NYC. This is why the wedding is outside in the street with no one around. Likely her family was unable to travel.