Can't remember exactly how the SP starting grid works, but I guess you're referring to loading up the grid in the exact player order every time. This would be highly unreliable in multiplayer unless you know you'll have the same people on every time.
You can use the start=finish/random/reverse in-game with a chat command too, but outside of that you will need to look into using InSim's grid reorganisation features.
// IS_REO : REOrder - this packet can be sent in either direction
// LFS sends one at the start of every race or qualifying session, listing the start order
// You can send one to LFS before a race start, to specify the starting order.
// It may be a good idea to avoid conflict by using /start=fixed (LFS setting).
// Alternatively, you can leave the LFS setting, but make sure you send your IS_REO
// AFTER you receive the SMALL_VTA (VoTe Action). LFS does its default grid reordering at
// the same time as it sends the SMALL_VTA and you can override this by sending an IS_REO.
struct IS_REO // REOrder (when race restarts after qualifying)
{
byte Size; // 36
byte Type; // ISP_REO
byte ReqI; // 0 unless this is a reply to an TINY_REO request
byte NumP; // number of players in race
byte PLID[32]; // all PLIDs in new order
};
Or features built into InSim addons such as Airio.