Late Night introduces a new career and a few new abilities. If you love the show business, the new Cinema career is great for you. You can choose to be an Actor or a Director, and according to the branch you'll follow, you get different career rewards, like the director's chair or the actor's van. This is a traditional career, not a profession, so you won't be able to follow your Sims at work, that is, in the Studios. 
As for abilities, beyond the new instruments (piano/keyboard, bass and drums) there is also the Mixology skill, that is, the ability of creating drinks and cocktails. It is not a new career as a bartender, but it's a normal ability that allows you to moonlight as a mixologist in clubs and bars. As we said, each drink has a different color and effect, and there's a lot of them that you can taste.

The bartender can offer drinks to other Sims (there is a dedicated menu option that sounds something like "Drinks on the house" for 48§ - at least in the example we saw. It may vary according to the number of Sims available), name a drink after someone (who knows if this will help for romance) and of course, he can do all the moves of the perfect cocktail maker (be careful, unexperienced bartenders can break a lot of glasses).

Drinks can be also prepared at home and placed in your Inventory or sold to the supermarket.

Bands

If you have Virtuoso Sims, they can join together and create bands, playing in different clubs. Late Night introduces three new instruments: the bass (well, it's more like a double bass, at least from what we've seen in one of the clubs), the drums and the piano/keyboard, each of which has its own relative skill. Unexperienced musicians will play for free, but gaining skills they can become great players and be well paid.

If you are a musician, you can register as an independent worker at the city hall, like you do for other abilities like writing or painting after Ambitions. It isn't part of the Rockstar career of the base game. 
In the club we saw during the event there was a small stage with instruments: musicians, in fact, will find their equipment right in the club, and won't use their personal instruments. Note: bands are pretty instrumental, as there is no microphone and you cannot sing.

As for the band, members can always join or leave (even though we don't know the maximum number of members a group can have). When making friends with someone, you can ask other Sims to be part of your band or kick someone out.

Groups

Groups can be created for different purposes, and although we don't know if there will be different kinds and names, we think it will be possible to create the coworkers group, for example, or the one of close friends, females only, or - why no - the group of celebrities! Wherever you want to go, if you invite the member of your group, they will follow you…if they can have enough fun! 
The members of your groups are indicated by a specific symbol: a light blue plumbob that rotates above their heads, and you can talk and have a drink with them even while sitting. If the members of your group throw a party, you can be invited and enjoy your time. Also, if the Sim that has the party is a celebrity, you will find the bouncer at his/her house.