When do you ask a professor if a visitor can sit in on class?

Question

There's a prospective graduate student visiting my adviser and I was asked if the student could come with me to an undergraduate class I'm taking at another college within the university. Is it acceptable for me to just show up to this class with the prospective student or should I email the professor and ask if the visitor can sit in? The class is lecture-based and doesn't require student discussion, relatively large (~40 students at both the undergrad and graduate level), but the professor will likely notice that the visitor isn't in the class.

Edit: I'm in the US and the prospective student and I are both from here, but the professor is from the Netherlands.

Update: I ended up emailing the professor three days in advance to make sure it was fine.

Answer 1

E-mail a few days in advance.

Just showing up puts everyone on the spot, and could end poorly. It could also turn your visitor off.

Answer 2

I suspect the right answer to this question depends very much on the country you are studying in.

To give one example, in Finland it is determined by law that all university lectures are public and free for anyone to attend. Nobody would notice an extra attendee, unless there were normally only a couple of students in the class. Emailing the professor would just seem odd.

Answer 3

I have taught a lot of small classes where I know most all of the students.

I also host office hours for my local Python group in a conference room, where people are expected to come and go at will, and new faces show up all the time.

In the former case, a new student showing up out of nowhere would have definitely raised my eyebrows, and an email in advance would have been sufficient warning to avoid any awkwardness. But in either situation, I would greet the new face warmly and welcome them.

Some schools may have more restrictions on access than others, and what you mostly want to ensure is that your guest has permission to be on campus, usually registering their presence at a visitor center before going elsewhere.