I've never seen the Terminal 5 Admirals Club to be that full. It's a good sized lounge (used to be the Delta Sky Club) and not a lot of AA flights leave from Terminal 5. I wouldn't think you'd have an issue getting into the T5 lounge, though would wait to buy entrance at the lounge.

The Terminal 4 Admirals Club can get pretty full as the bulk of AA's flights leave either from there or TBIT which is closer to T4.