My pet tank and I were about halfway through the Phantasmal Fortress lair quest from Dread Ring when suddenly, a scourge warlock ran past us. "Where did that guy come from?" I wondered - after all, it's an instanced quest. Then I saw that according to the UI he was actually part of our group.
I tend to forget that Neverwinter actually allows open grouping by default since I so rarely see it happen. You can manually "lock" your group to prevent others from joining without permission, however you have to re-apply this setting every time you form a party. As far as I can see there is no way of making it so that your groups are locked by default whenever you create one.
My pet tank was happy that the extra damage dealer made things go faster, but I felt slightly "invaded". It's one thing to co-operate with random strangers out in the world, but having one suddenly join what was meant to be a private party feels uncomfortable to me. I wonder if that's something I could get used to with more practice.