For big groups, dancing works. It's a bit funny when they're not in a nightclub or in one of those spontaneous parties GT has, but it works pretty great to build multiple relationships all at once.
...Even if you don't talk to anyone, as long as you're all dancing to the same thing it works pretty great lmao
I'm not too clear for stereo vs the DJ booth, but I'm guessing the latter has that effect, as well as dancing together on the dance floor. My sims haven't been partying since they had their last kid. Multiple toddlers are hard work, hehe