Green Arrow, Atom and Hawkman were the first 3 heroes to join the JLA after it formed...
...and here's the facts on them and when these solo heroes became Leaguers as echoed by the stories under this classic cover of Justice League of America #67 by Neal Adams!
Speedy (Roy Harper) moved over to World's Finest Comics (making a cover outside of the JLA only with Brave and the Bold #50, with the Martian Manhunter; he even had few foes coming to face him in the JLA, like Clock King).
Chronos, the Highway Man, the Bat-Knights, the Bug-Eyed Bandit, the Panther Gang, Dr. Light, the Thinker as well as socializing with other heroes like the Earth-2 Atom, Zatanna and Hawkman and Hawkgirl, and even gaining a pet in Major Mynah, and traveling through time with Alpheus Hyatt's Time Pool! All this was enough to keep the little man busy through 38 of his own issues (through August-September, 1968), then to a shared magazine with Hawkman (more on that later, but it picked up Atom's numbering, and Atom & Hawkman went to issue #45 in October-November, 1969, with the pair facing off against Jean Loring!).
Hawkman seems to have it all going on...sci-fi hero, police detective, ancient weapon wielder, devoted husband, and he did!