Thalmor:You can join by encountering a Thalmor Justiciar who will take you to Elenwen if you don't start the A Blade In The Dark quest

The Blades:Only joinable by killing Firstborn Alduin,who,while isn't super-nice,isn't a jerk

Imperials:The Legion of Tamriel.
Servers Of 8:The Imperial religion

Talos' Guard:The Freecloak religion
Freecloaks:The army led by Galmar Stonefist

The Dark Brotherhood:The best way to reach the Dovahkiin Listener