Not every part of the Invasion! was covered in official crossovers, or its own 3 issues, and here are a few of the secret tales that filled in a few holes in Invasion!, and helped to place a few missing members of the DC Universe in one of its biggest events.
Suicide Squad and TitansBronze Tiger, Vixen and Duchess facing the Warlords of Okaara and Thanagarians in Russia with the Rocket Red Brigade, Nemesis and Captain Boomerang fighting Khunds in Australia, and the introduction of computer guru Oracle (off-panel). With New Titans #52 (January, 1989 by Marv Wolfman, George Perez and Bob McLeod), Speedy and Danny Chase were trying to find Titans to help in the Invasion! while the active members of the team (including Nightwing, Cyborg, Starfire, Jericho, Changling and Raven) were in space, finding out the history of Donna Troy/Wonder Girl.
Hawk and Dove and Power Company?
First Strike Aftermath
Atom and Superman
Action Comics WeeklyGreen Lantern, Black Canary, the Demon, Phantom Stranger, Deadman and Speedy were in various of the crossover issues....and Wild Dog even had a cameo in a panel in Invasion! #2!
L.E.G.I.O.N. and Justice League Europe
Better still, the Suicide Squad story was recently reprinted in Suicide Squad: Rogues in 2016, the New Titans story reprinted in New Titans: Who Is Donna Troy tradepaperback of 2005 , and Justice League Europe #1 can be found in Justice League International tradepaperback #5 of 2011, so it should be easy to find these stories, along with the tales around them to add more meaning!