As I recall-
The Pentecostal Outpouring was mainly among those already involved in The Holiness Movement
William Seymour picked a Nazarene (Holiness) church first in Los Angeles, and was asked to leave as I recall. He then went to the house on Bonnie Brae (help me here someone) and the crowd grew so large they eventually broke the porch from the weight of the bystanders. They then looked for a larger meeting hall and found the former stable (again help me here someone) on Azusa Street.
Here is a site with a timeline