@crinrict, I think this is a bug having to do with festivals. After reading @NZsimm3r 's post, I played this morning and again didn't have a problem having my sim sing a duet with a sim in her group.
This was in an older save, so I cleared cache and played a save I started after installing C&D. When I grouped sims and took them to Planet Honey Pop after declining an invitation to go to the Romance Festival, my sim couldn't sing a duet with another sim in the group as they didn't appear on the picker menu. What I hadn't taken into consideration was that Planet Honey Pop is across from the festival and the game must have thought I had gone to the festival because my sim had the option to "throw petals". I even had the sims stay until the festival was over and still couldn't choose anyone from the group to sing with. It didn't even help to disband the group and regroup them. I was going to try the other karaoke venue but unfortunately, a vampire entered the lot and my sim couldn't travel.
The next time I grouped sims to go to the Waterside Warble karaoke venue, there was no festival and again, I was able to choose a sim to sing a duet with from the group. I'm not sure if the bug would be at both karaoke venues at the same time if a festival is in progress across from one of them since I couldn't travel due to the vampire incident. I'll test that when the next opportunity arises.