[Denying]

First, you will need the following under your #includes

pawn Код:

strtok(const string[], &index)
{
    new length = strlen(string);
    while ((index < length) && (string[index] <= ' '))
    {
        index++;
    }

    new offset = index;
    new result[20];
    while ((index < length) && (string[index] > ' ') && ((index - offset) < (sizeof(result) - 1)))
    {
        result[index - offset] = string[index];
        index++;
    }
    result[index - offset] = EOS;
    return result;
}

And copy the following OnPlayerCommandText CallBack.
public OnPlayerCommandText(playerid, cmdtext[])

pawn Код:

{
    new cmd[128], idx;
    cmd = strtok(cmdtext, idx);

    if (strcmp("/cp", cmdtext, true, 10) == 0)
    {
        new targetplayer[MAX_PLAYER_NAME];
        targetplayer = strtok(cmdtext, idx);
        if(strlen(targetplayer) == 0) return SendClientMessage(playerid, -1, "USAGE: /cp [playerid]");
        if(!IsPlayerInAnyVehicle(strval(targetplayer))) return SendClientMessage(playerid, -1, "ERROR: Chosen player is not in any vehicle!");
        PutPlayerInVehicle(playerid, GetPlayerVehicleID(strval(targetplayer)), 1);
        SendClientMessage(playerid, -1, "(Info): You have successfuly teleported into a player's vehicle.");
        return 1;
    }
    return 0;
}

Should work, not tested.

NOTE: This may be buggy as it will always TP the player inside the front seat. ( Near the driver )
And if that seat is taken then..