Are Shared Office Common for Lecturers
You're going to find answers all over the map for this one (i.e., there probably aren't "standards").  I personally like having a private office so I can get the most work done, but I'm generally comfortable with a shared space if it doesn't get too loud.  I have found that it is rare for students to worry about how they perform if they are just trying to get help figuring out the material.  If there are conference rooms or open classrooms available, that's sometimes a good place to tutor students, too.

For what it's worth, I never have closed door meetings with students because that has the potential to lead to allegations of improper relationships, etc.  If a student does have something private to discuss, I'll ask them to take a walk around campus where we are visible but can't be heard, or to go into a conference room that has windows into the hallway.