So I built a new nightclub and noticed that all the sims show up wearing their casual clothes. Is there a way to change this to e.g. party/formal wear?

You need the Formal Outfits At Lounges (1.19.31) mod by Shimrod. Works perfectly for what you want.

A nice mod, yet it's only specific to the lounges.

I remember, easily over a year ago, I had my own mod that made player sims change into party wear for nightclub venues. Also my own for formal at the lounges. Then Maxis did a massive rework on the venues with all the new Zone Director XMLs, so I tossed a lot of my old, broken venue mods.

Recently, after Shimrod released the formal dress at longue one, it reminded me of my old party dress for nightclubs mod. I took a look to see if I could recreate, and boy, is it so not happening. The amount of "barfly" jobs for nightclubs, I just wouldn't know where to begin to hit the right one that would get them all in party dress for that. There's so many!

I suppose if I knuckled down and faffed about with them all, I'd hit the right ones eventually. But then the real question is, how badly do I want them in party dress at nightclubs to go wading through all that? Currently, not that badly. It would have to be a Frankenstein number, surgically crowbarring the dress coding data into the XMLs as I just do not see it where it once was, and hoping it doesn't turn out to be a monster. Because you gotta ask why did Maxis go the trouble of taking it out? It used to be a simple matter of changing a false to true for player sims to dress in the same way NPC townies did. Now that's all gone. As well as an abundance of barfly types to fathom.

But I wouldn't say it's impossible.