The first additions to their ranks happened very soon after they were formed...
Flash and Hawkman, Johnny continued to adventures in Flash Comics (as well as in the New York World's Fair #2, World's Best Comics #1, World's Finest Comics #2 and #3, and later the Big All-American Comic Book, and even appearing in All-Star Comics #2, and guesting with the JSA in All-Star Comics #3 and #4 (and a solo text adventure in All-Star Comics #5) before officially joining the JSA with All-Star Comics #6 (August-September, 1941). It's a good thing Johnny joined the JSA, or he'd barely be noticed...Johnny and his Thunderbolt didn't end up on many Flash Comics (or any other comics) covers, except for All-Star Comics.
Mr. and Mrs. Superman feature), and Johnny returned with the JSA in Flash #137...though the Thunderbolt didn't return until Justice League of America #37, wherein it was revealed that Johnny had an evil Earth-1 duplicate, who also used the Thunderbolt for a time, and this evil Johnny also returned in Justice League of America #38, #219 and #220 (wherein more of Black Canary's history was revealed as well).
Man of the Stars
original Black Canary in two issues of Brave and the Bold (#61 and #62 in 1965, the first also reviving an old foe of Ted's, the Mist), and continued to work with the Justice Society on and off until just after the Crisis on Infinite Earths, which took most of the team away for a while...
Doctor In The Dark
Martian Manhunter in Detective Comics), and Wonder Woman was there for the revival of the JSA in Flash #137 (June, 1963), and the World War II Wonder Woman even had a revival in Wonder Woman (in Wonder Woman #228-243, DC Special Series #9, All-New Collector's Edition #C-54, and World's Finest Comics #244-250 in the 1970s), due to the Wonder Woman TV show with Lynda Carter, taking the Wonder Woman comic over with Wonder Woman #228 (and meeting her Silver Age counterpart and fighting the first Red Panzer, Nazi Helmut Streicher who was there at the formation of the JSA in DC Special #29 and whose later incarnations would fight Wonder Girl Donna Troy), keeping the book until Wonder Woman #243 (May, 1978) with another team-up and switching back the title to the Silver Age Wonder Woman. After these appearances, differences between the Wonder Woman were established, such as the Golden Age's Wonder Woman's mother, Hippolyte, being brunette (and the Silver Age Hippolyta being blond), and the Golden Age Wonder Woman had married Steve Trevor, and even had a daughter, Lyta Trevor (destined to become Fury of Infinity, Inc.).
Freedom Fighters and more) in tales set in the 1940s (but presented in the 1980s) as well as being a frequent guest in Infinity, Inc....at least until the Crisis On Infinite Earths ended her career (though the Golden Age Wonder Woman popped up a little to help the current Wonder Woman during Infinite Crisis). All this history shows why Wonder Woman needed a Wonder Woman Encyclopedia!