You have to "hike" to the corkscrew. It is all open seating up there...just pay general admission. You either stand or sit in the dirt. There are some bleachers but they are not right at the track. The only downside to being out in the general areas is that you cannot see the video screens.
To meet the riders during MotoGP you have to buy Paddock Passes.
I am sure it is MUCH more packed during MotoGP (everywhere else is!
) but during last years AMA finale race we were able to stand at the fence all around the corkscrew and Rainey Curve.
HIGHLY recommend you do all your walking around/shopping on Friday or Saturday. Sundays during MotoGP are packed!
If you want to get a good general seat you will have to guard it all day Sunday.