Who's This? The evil Justice League.
How you could have heard of them: Still around after Infinite Frontier, the "CSA" recently appeared in Suicide Squad (after figuring in the team's Future State story), leading to an event dubbed the "War for Earth-3" which absorbs a few of the Syndicate's members into the Squad. The group also appears in the animated Justice League: Crisis on Two Earths release.
Example story: The Secret Society of Super-Villains #13-14 (February-May 1978) "One Earth too Many" and "Crisis on Earth-3 (Reprise)" by Gerry Conway, Mike Vosburg and Bob Smith
The Wizard has done his own research, and concluded that if they have three tokens of power, they can pierce the dimensional barrier again and finally reach Earth-2. Those tokens? Super-Woman's lasso! Power Ring's, uhm, power ring! And Johnny Quick's speed helmet! (Who's Who's writers either missed this story or chose to ignore it because Quick's powers are listed as coming from an undisclosed source.) The Wizard even uses his abilities to hack the power ring and send the Syndicate a challenge to "meet us separately". If the Syndicate has any common denominator, it's that a life of unchallenged tyrannical rule over their Earth has made them cocky enough to accept the invitation. First bout: Johnny Quick vs. the Floronic Man (at this point, "Plant Master"):
Jump successful! And I didn't even get lost on the wrong Earth in the process. To answer my own question, it seems that Johnny had a spare helmet (so what's he complaining about? the color?!), Power Ring has a lantern through which he can channel the same powers (this is new), and Super-Woman flies and has some measure of super-strength (she IS a renegade Amazon, after all).
It's not a great showing for the Syndicators, but then it's always felt like Ultraman and Owl-Man were carrying the team. This sad chapter might well bear this out. Heck, Super-Woman didn't even get to transform her lasso into shapes like some kind of villainous birthday party magician. Coolness factor of Who's Who entry vs. actual field experience... the gap is dramatic.
Who's Next? DC's first masked hero.