The player lives in a town (named by you) who participates in festivals so they can unlock more residents and houses in their town. MC Emi and her partner Mitch are the organizers of the festivals. Festivals are collections of minigames ran by a certain host. The minigames are themed to whatever a festival is themed to, like how the Music Festival has minigames related to music. At the end of each minigame, a handful of points are handed to each competitor, with the winner having the most points given and the runner-ups given some points for effort. At the end of the festival, the competitor who has gathered the most points throughout the festival wins. Monuments, statues that can be placed around town, are won when reaching a certain amount of points. You can also get decorations for your house and also unlock a few things in Create-A-Sim along the way.
The first festival is organized by Buddy which has 3 minigames which you have to play to unlock the next festival, the Happy Festival). Each festival has a name and the host usually has to do something with the type of festival it is, like Dr. F is the host for the Bizarre Festival and DJ Candy Supergroove is the host for the Music Festival. There are 12 festivals in the game in all.
When participating in festivals, you can have a team of Sims. Festivals unlocked in the beginning usually only allow 2 team members, but as the player experiences festivals later in the game, there are festivals that allow up to 4 team members per team. On each minigame you participate in, you select one team member from your team. On each minigame, the stats used for the minigame that was played usually go down, so it is recommended by the game to switch between team members instead of using one Sim the whole time.
MySims Party has more than 50 minigames. There's a stat system with 4 basic stats: Power, Speed, Stamina, and Luck. Each minigame includes a vital stat that can benefit a player that has alot of it. For example, having a lot of Speed in a Speed minigame will make you have an advantage over other players with lower Speed.
The town is simply a place outside of festivals where you can explore, but you can't do too much. You can change your Sim's clothes, and change the exterior of your house. You can also place monuments you've earned from winning festivals and talk to the residents of your town, who may give a tip or two about the minigame they host. Residents move in once you've reached at least 60 points in their minigame. You also talk to MC Emi at the center of town to enter festivals.
MySims Party only has 51 of the original characters from MySims. Although many characters are not in the game, you may create as many "new" (custom) characters as you want. You may also recreate the original Sims that you want to play as. The only new characters to the series are MC Emi and Mitch.
Each and every character runs one minigame, and a few run festivals. You can unlock them as playable characters if you've reached at least 80 points in the minigame they've hosted. They will also move in your town if you've reached at least 60 points in their minigame.