Ragged Flagon if they count. There is the Forsworn group north of Karthwasten, they become non hostile if you side with Madanach in the mines. It is quite a good camp, wheat mill, blacksmith equipment, good place if you just want to hang out, can upgrade your equipment while your doing it. The Alik'R group in Swindlers den. If you don't complete the quest they won't turn hostile , you can hang about there. Non hostile bandits wold be quite a find. The friendliest I've had from a bandit is a warning not to get any closer. There is one cave with three Hagravens and a bandit in a cage. He begs you to rescue him, helps you clear out the Hagravens then when that's finished he turns hostile and tries to rob you.
Druadach Redoubt might be the best place. They will not turn hostile if you help Madanach escape. Even if none of the Forsworn from the mine survive the attack on Markarth. http://www.uesp.net/wiki/Skyrim:Druadach_Redoubt