Bonnie only needed a little convincing, and then she was like OKAY YES. Because she loves karaoke and she deserves a few drinks dammit. She can sing. She probably did a few. TLC's No Scrubs among other pop and/or R&B songs.
Sara Lance went! Her voice isn't super powerful, but she can carry a tune well enough and has zero fucks so she pulls it off. Since she can't actually read the lyrics on-screen this is totally by memory. Meredith Brooks, Bitch, and just anything really stereotypically karaoke. She was here to have fun.