[Iguman]

I think that i've fixed your errors, but from what I can see, you aren't inviting the player but setting his faction and rank right away.

PHP Code:

ReturnFactionName(playerid)
{
    new string[64];
    switch(PlayerInfo[playerid][pFaction])
    {
        case 1: string = "State Police";
        case 2: string = "Federal Security Directorate";
        default: string = "Undentified";
    }
    return string;
}
CMD:invite(playerid, params[])
{
    new id;
    if(PlayerInfo[playerid][pFaction] < 1) return SendClientMessage(playerid, COLOR_RED, "You are not in a faction");
    if(sscanf(params,"u", id)) return SendClientMessage(playerid, -1,"USAGE: /invite [ID]");
    if(!IsPlayerConnected(id)) return SendClientMessage(playerid, COLOR_RED, "ERROR: Player is not connected!");
    PlayerInfo[id][pFaction] = PlayerInfo[playerid][pFaction]; // This should be done after player accept the invite
    PlayerInfo[id][pFacRanks] = 1; // As well as this 
    SendClientMessageEx(targetid, COLOR_AQUA, "%s has invited you to join the %s", ReturnName(playerid), ReturnFactionName(playerid));
    SendClientMessageEx(playerid, COLOR_AQUA, "You have invited %s to %s", ReturnName(id), ReturnFactionName(playerid));
    return 1;
}