LOL I just had a very low level port to me in the battle with the monkey king doppelgangers. She was level 12. She came in alone and added one mob. Since she was already in the battle; I told her to run to the bottom my side. She did and the extra mob went for me and I killed all.
She got over 1k gold and a level 40 robe that she can use in 28 more levels.
I do not think this change was made so that lower levels can actually farm very high level mobs that easily.
I also think there needs to be a port option so friends can not port to you sometimes.
Can't help with the low levels porting in to farm, but I can help with them porting at all. Go to the options part on your screen, find the key, and change friend teleports to "block" when you don't want them coming into your fights.
In the second game play tab you may also make yourself invisible to friends. Just remember to toggle it back to visible when you're feeling more sociable.