in the beginning it was just him.. he would never ask any of his brothers to follow him and leave their family for him.. but then while he was nearly to the Cullens' house,he realized Seth Clearwater followed him..the latter could not accept to destroy bella and probably the Cullens.. he thought of them as people,good people,friends.. then after a while Leah Clearwater,seth's older sister followed and joined jacob.
Seth left first cause he didnt want to fight the cullens either, then his sister Leah as there would be less people who listen to her thoughts and she could get away from Sam aswell as to protect her brother. Embry and Quil both later joined Jacobs pack aswell after it was found Renesme was harmless
Seth went with Edward due to the fact that he did not want to fight the Cullens and Leah came later she couldn't be around Sam and Emily better off with Jacob and she could look after Seth even though she didn't care for the Cullens at all