[Makaveli93]

pawn Код:

new string[512]

for(new i=0;i<MAX_PLAYERS;i++)
{
    if(PlayerIsACop[i]) // insert your own check here
    {
        new pname[MAX_PLAYER_NAME];
        GetPlayerName(i, pname, sizeof(pname));
        format(string, 512, "%s%s\n", string, pname);
    }
}

ShowPlayerDialog(playerid, 5, DIALOG_STYLE_MSGBOX, "Police officers online", string, "Okay", "Close");

Just a rough example.

Now, for checking offline members you would have to use MySQL or have some kind of storing system for faction members, maybe storing their names in a file?