I want to know how Klaus possessed Alaric & how he seems to have his powers like being able to compell other vampires
Answer on A question about the vampire diaries?
The male witch buddy of Klaus' transfused some of Klaus' blood into Alaric and did a spell that allowed Klaus to possess Alaric. Because of the spell, Klaus, in Alaric's body, retains some of his powers, so he can compel people (and vamps). He can enter homes without an invite since he's in a human body, but he doesn't have any of his "physical" powers such as super speed and rapid healing.