Honestly the solution here is "only play with people who you know are willing to cooperate and share." Just making it so a stranger you invited to your party can't take all the loot doesn't change the fact that they can still trigger cutscenes, choose dialogue options, and otherwise make changes to your game you may not be happy with. So many of the complaints about multiplayer in these forums seems to boil down to people letting jerks into their games. Larian can't help you there. :\
For those who have friendly players, here's a technique that ensures fair and equal distribution. Have everyone go around the area looting what you can. When everyone's satisfied that everything has been picked up, one person sends the mundane items and junk to their camp. They are the junkmonger. (They should be someone OTHER than the person with the highest CHA score.) Sort the scrolls, potions, and arrows to the characters/containers that hold them*. Then put all the magic items obtained in one character's inventory so all players can review. Talk amongst yourselves and divvy up the loot as you see fit. Any items that nobody wants also get sent to the junkmonger's camp.
*Assuming you have dedicated characters who hold such items. If not, divvy them up with the rest of the loot.
The Junkmonger should have multiple storage items in their camp box. One Camp Supplies bag (so you don't have to carry that weight around all the time), one container of items that you want to sell, and one container of items you want to keep. You can have more containers but you should have at least those. I also have containers for bag overflow (extra potions/arrows/kits), rotten food (we will eventually sell it all!), a container of containers, and a barrel of substance-filled barrels. The "items to keep" container is split amongst mundane items, magical items, books, former quest items, and things I think may secretly be quest items even though they're not marked as such. (I probably failed to pick up the relevant quest.) It's important to keep all this in the camp chest so you can access it from any camp. I used to keep it all in containers out on the ground in camp, but then if your location sends you to a different version of camp, you're out of luck.
When you find a merchant, have the person with the highest Charisma talk to the merchant. Send the Junkmonger back to camp, and have them open the chest of stuff to sell. Highlight all of it and drag it onto the person talking to the merchant. You will probably far over-burden them, but as long as you drag it all in one go, you can move it all over.
My husband and I finished Act 2 with 20k gold, plus a chest full of magical items that is worth more than 20k more. I'm guessing well finish Act 3 in the 50k range. And that's even though we buy scrolls, alchemical ingredients, arrows, and items from every merchant we find. We also murder about 75% of everyone we meet and loot every city bare, so that helps. >:)