Congrats! It depends, but usually at least half. If you are invited out this early then it is probably higher than that.
Be yourself. The campus visit is a great opportunity for you to interview them as well. You want to find out everything about the school that you can. Ask the PhD students about the program and interactions with faculty. Ask professors what they are currently working on. You should try to get an idea of this yourself, but it is also a good question. You will probably tell your "why a phd" story several times.
Have you ever attended a workshop? Read the paper beforehand so you can try and understand what is going on. Be attentive, silence your phone, etc. You shouldn't be afraid to ask a question or two if you have them but you are probably not going to impress anyone with your insight at this point. It would be perfectly fine to be silent throughout. If you ask any question that isn't completely stupid, that would be a positive. It would be bad if you fall asleep, text the whole time, or talk so much that you derail things (I have never seen an interviewing applicant do this).