new bool:ssdm[MAX_PLAYERS];

if(!strcmp(cmdtext,"/dm1", true))
{
    ssdm[playerid] = true;
    //SetPlayerPos(playerid, X, Y, Z); The pos X Y Z you want to teleport the player...
    return true;
}
if(!strcmp(cmdtext,"/leavedm", true))
{
    ssdm[playerid] = false;
    //SetPlayerPos(playerid, X, Y, Z); Pos X Y Z to exit minigame...
    return true;
}