[Threshold]

If you want to make them completely invisible, then you can set their colour to something but with a complete transparency value.

Red (Fully Visible): SetPlayerColor(playerid, 0xFF0000FF);
Red (Invisible On Map): SetPlayerColor(playerid, 0xFF000000);

I haven't actually experimented with these as much as I should have, but I think this should work in all honesty. Or you can always use SetPlayerMarkerForPlayer as an alternative.