Renly doesn't spend much time with these supporters before he's assassinated. Still, it's not the amount of time but the quality of that time that really counts, right? On second thought, that might not be the case, especially if you look at things from Renly's perspective:
Ser Cortnay Penrose is the bold and brutally honest castellan of Storm's End in Renly's absence. After Renly's death, Penrose refuses to hand Edric Storm over and instead challenges Stannis to single-combat. That night, Melisandre births a second shadow assassin to murder Penrose. Single-combat against a magical assassin? Probably not what Penrose had in mind.
Edric is Robert Baratheon's illegitimate son. Stannis takes Storm's End mainly to acquire this boy, whom he plans to parade around King's Landing to prove Joffrey is not Robert's heir (11.Davos.106). Because nothing proves paternity more than a cursory glance of facial features—just ask anybody on any daytime talk show ever.
Ser Loras Tyrell is the Knight of Flowers and served as the head of Renly's Rainbow Guard. Catelyn has "heard tales of the prowess" (23.Catelyn.50) Loras displays in battle, but even this cannot keep him from being toppled by Brienne during their melee. After Renly's death, Loras joins Tywin with the rest of the Tyrell forces to rout Stannis's forces at the Battle of the Blackwater.
Margaery Tyrell was Renly's queen before he was assassinated. After the Tyrells help Tywin fight off Stannis's forces, she becomes engaged to King Joffrey. Not her lucky day, we're thinking.
Mace is the patriarch of the Tyrell family and the Lord of Highgarden. He brokers a deal with Littlefinger to support the Lannisters against Stannis's forces, and after the battle, Mace sees his daughter betrothed to the king and even gets a seat on the small council to boot.
Randyll is a stern bannerman of House Tyrell. He is part of Renly's forces and later joins with the Tyrells fighting alongside Tywin at the Battle of the Blackwater. We bring this guy up because he's Sam's father. Yeah, the same Sam currently freezing his butt off beyond the Wall with Lord Commander Mormont. It's absolutely not a small world.