If this is a statement all on it's own, it probably refers to whether you can "count someone in" or not. That is are they going to participate or join in. "I'm so in." would you usually have the emphasis on the word "so", indicating that the person can definitely be counted in or will eagerly participate. If someone said "We are thinking of hitting the clubs tonight. We heared that there may be some celebrities around. Are you coming?" Someone might respond "I'm so in."

A couple of possible meanings come to mind: "I am very much in [style][the elite]" "I am very much inside this group/organization" [The group or organization is hard to get into] "I am [physically] very much inside this building/room." The phrase comes from "I am so .... that...." Example: I am so tall that I have to duck my head when I come in the room.