Margo went along with El and crew and planned to drink and catcall mostly, but wasn't that hard to drag on stage either, she just let El take the lead. She's not a great singer, but she gets by. She let El pick the song but told him if he picked something that made her sound bad she'd kill him.
Will went along with the Losers and kind of surprised himself (after a little of Bev's weed) by going up and kind of turned into a surprising ham on stage. He does "Should I Stay or Should I Go" and while he's not SKILLED, he's not BAD either.
Alex went. He'd have asked Michael & Maria if they wanted to go, but gone alone if not, and Alex can sing. He did some Panic at the Disco or Fallout Boy or something, 'cause emo rock history.
Geralt did not sing, but probably went if Jaskier did.
Grog maybe went to sing the Eddie's dick song for Stan. Not on stage. Probably? I dunno. He might have just ended up there because the stage was shiny and then sung one of Scanlan's songs out of nostalgia and gotten weepy in a big ale.