Any fast prime will do...
Cheapish and good: Tamron 17/3.5 (no fast option), Zuiko 28/2, Rokkor 58/1.2, Canon 135L
If you're in a budget, replace Rokkor and Canon 135 with Zuiko 55/1.2 and Samyang 85/1.4
If you want to spend loads of money, add a Distagon 35/1.4 and/or 24L II
Less lenses the better, one wide and one for head/shoulders with a slight bit of compression. So 28-35 and 70-100 range lenses, IMO. Big aperture so no flash is necessary and it throws the usual ugly background into a mush of sorts. Sharpness is highly tertiary, as everyone's going to remember the party a little bit fuzzy anyhow