You can always get in as a plus one, or if you have a "sponsor" with enough clout to score an invtie for you. But it has to be an invite. First, all the nominees and their guests get invites, and there's a hierarchy to it. Like Directors get maybe +3 or producers get +3 where as an acting nominee gets a plus one.
The presenters get a plus one. Sometimes there's trading, for example, if Meryl wants her entire family to be there, they will "find" tickets someone else isn't using. But Meryl wouldn't do that.
(I often wonder if there is a sort of separate invitation for the Governor's Ball or if it is automatic that you get in if you are at the awards ceremony.)
Then there are the Board of Governors of the academy, and other VIP types. Then there are studio moguls, and bigshots and
definitely the top honchos of the major agencies like CAA, WME, etc. who buy in.
If any AMPAS member wants to buy tickets, they can come. But it's very closed and very controlled. BTW: Did I tell you you have to buy your tickets?
Top PR firms who handle stars also finagle their publicists to "work" the event, so you see them strutting around the red carpet with walkie talkies steering Celebs towards media for interviews, and trying to look important.
Everyone is posing and posturing. The major film people are fine. It's the publicists, security people, and PAs who manage to get on everyone's nerves.
Even agents get pushy holding their nominees' hand to try to secure their place in the firmament. Often a celeb will insist their own publicist or PA be on the carpet, to "assist " them. Like it's so hard to walk the carpet.