The flights I've been on have had five groups. The first has no group number on the boarding pass (I just checked one of mine); they look at your row number to see if you're in first or business. (On large aircraft they board these separately.) Group 1 is elites flying in economy. Groups 2 through 4 are non-elites in economy, from the rear of the plane forward.

There have been several discussions here about groups traveling together when some are elite, some aren't. Apparently AA has no consistent policy on this.