The Rio (GIG) Admirals Club is open daily from 11:00am until 8:30pm, local time. Only ticketed departing passengers can visit the club:
"The Rio de Janeiro Admirals Club is located in the Blue Sector restricted area, Mezzanine Level, above Gate 8."
For more info, see:
http://www.aa.com/content/travelInfo...eJaneiro.jhtml
Most international clubs (with the major exception of FRA) are located inside security and exit immigration, meaning that you must be a departing passenger in order to use the club.