I second the recommendation to add more lowbies - you must add lowbies lower or equal than the one you care about - the best number (from experimenting with all combination) is 3 lowbies and the 1 high level - if you have level 70 or so it probably will yield more xp too but I don't know what the difference 70->80 is.
So if your friend is l17 or so try to get 2 level 15 to join and he'll get a lot more xp; try to keep the "add's 2 levels or lower below - ideal being level 15 (min level for stocks). A level 25 and 2 level 15 will really sponge the xp for instance

ps: stocks will give more xp than sm until 25+