pawn Код:
SetPVarInt(playerid, "Invited_By", -1);
SetPVarInt(playerid, "Group_ID", -1);
You are checking for INVALID id types when player uses the command /accept.
So that code above must be:
pawn Код:
SetPVarInt(playerid, "Invited_By", INVALID_PLAYER_ID);
SetPVarInt(playerid, "Group_ID", INVALID_GROUP_ID);