[Gammix]

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);