Disclaimer: cloud variables are broken, so it won't work. This project is the second version of Multiplayer Test (https://scratch.mit.edu/projects/870224002) and hopefully it works better. I tried to fix the many bugs of the older one, but I haven't tested it out yet to see if it works. Up arrow key or w to go forward, right and left arrow keys or d and a to turn, and down arrow key or s to go backwards. This can only hold two players at a time, (sadly) so if you can't get in, it means two people are already playing.
All the code is by me, except the "when stop clicked" code. I changed the "when stop clicked" code because whenever you press the green flag twice, the second time is counted as (scratch's timer) > (timer the variable). Fun fact: this project has 108 blocks. The actual multiplayer code has 103 blocks.