scripting question

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hey, im making a spar complex system where when you enter the level, or say /spar, a gui comes up that says "Do you want to spar?" and there is a "yes" and "no" button. If you say yes, it warps you into the middle of the spar, and if you say no, it closes the gui. My question is, how can I make the script check if there is already 2 players in the spar? like in a certain x,y? How can I make them check for 2 players in that x,y? Right now, people just come in, while others are sparring. THanks, Jamerson