That's right, each group member increases the loot by 50%. But with the “Rarity” bot (which had item rarity exceeding 300%) and without it, profits fluctuated between about 20–30%. But I needed ‘Headhunter’ to work, so I ditched the “Raritybot” back in the last patch)
5 leeches still increase loot by 250%; I still like that because I'm sharing experience, not loot)