How to send data between systems during multiplayer?

Is it even possible to send data, like arrays, between systems like that? How can I keep track of the other player's XY position and such?

You can use MPSET index,value to set a multiplayer variable which can be accessed by other systems using MPGET(systemID,index,value) Index can be from 0 to 8 (yes, not 7), and the value has to be an integer. You can also use MPSEND string$ to send more data, and MPRECV OUT Id,string$ (only accept the string if Id is not -1)