It still has some bugs, but I think that the basics of the MMO engine work. MMO stands for Massive Multiplayer Online, and in theory, it should be able to deal with infinite online players, but scratch has the 256 character limit on cloud variables, so there is a player limit. How it works, is instead of having the players using the same variables for every frame, it makes the player use the variable once in each 5 frames, or more, if there are more than 10 players. Instead of loading the data from that frame only, it loads the data from all the frames since it was last updated.
If you want to use it, juat backpack the MMO sprite, go to the "Add data" block, and follow the instructions. (Make sure to give credit.) Devlog: v1.0 : Initial release, unshared after because it had some important bugs, and features that needed to be added. v1.1 : Fixed bug where you would see the player and his last frames. v1.2 : Added an extra time for when the players don't send their data, so they don't disappear for 1 or 2 frames. v1.3 : Made it easier to use, and shared it. v1.4 : fixed bug when only 2 people are online, and the other leaves. All here is made by me.