Sims of a feather.
The Sims series has never been adept at group control—a party with random strangers doing wacky, random things, yes, but not maneuvering a small gathering of people in tandem from one activity to another. The best you might be able to muster is controlling a household of Sims (say, for a family game night) by painstakingly pausing, clicking, and rotating between all of them. Corralling a flock of crazed cuccos into a pen might be easier.
The Sims 4 Get Together, as the name implies, attempts to fix this once and for all. It introduces the concept of clubs, a members-only group that can be rallied together at a gathering to do, well, pretty much anything you can imagine: a Glee club full of high-school students who play instruments, a group of douchebag bachelors tearing through the local nightlife, a fiendish gaggle of supervillains and ne'er-do-wells who prank and cause mayhem with everyone they pass. I settled upon a DC/Marvel-crossover superhero group that included Hawkeye, Superman, and Wolverine who would work out, eat, dance, and mete out justice upon the aforementioned supervillain group.
Over time, clubs can earn specific perks that strengthen the comradery between members and provide them with passive buffs, much like how retail stores in the Get to Work expansion can spend points to improve sales. So long as your members perform any of five possible actions (which you can change at any time, so it's really whenever your members do anything), the group's bank of club points will steadily increase. You can also set all sorts of banned actions, parameters, and requirements to ensure that your group is perfectly tailored to your specifications and preferences, like a Top Chef VIP fan club whose members must have baseline skill of Cooking at 2 or above, always meet at your state-of-the-art kitchen, and are filthy rich. If you wish to join a pre-existing club, you'll need to impress the club members enough to earn a spot, and if you gather enough influence, you can even take over as the leader in a coup (drama not included).
The most powerful perks given by clubs is the ability to control moodlets through emotional vibes, granting an unyielding +3 bonus to any particular moodlet you desire for the entire group. On top of that, you can increase the rate of earned experience for any available skill, which makes clubs an expedient way to max out every skill in the game (if you haven't done so already). Other perks give your club more options for exclusivity, like banners, sigils, secret handshakes, jacket uniforms, and a door that can lock out rival clubs from entering.
Apart from that, Get Together doesn't add too much content beyond two new skills, dancing and DJ, as well as the new European-inspired Windenburg. (Diving platforms are cool too, I suppose.) Where dancing is essentially a passive skill that maxes out at five levels and does little else, the DJ skill can get a crowd pumping (DJ Victoriam bringin' down the haus!). The higher your mixing skills, the more genres you have at your disposal and the better you can influence the crowd with happiness and other moodlets. At its highest levels, you can earn small amounts of tips and create mixtapes for royalties much like other instruments, though you're still limited to having only one royalty check per instrument for some reason. Wittenburg is a fine addition as well, with multiple nightclubs and cafés along a calm waterfront.