Well, it is inevitable to join the group (or should I say, you would feel you would like to) when you are in the group. That is to say that, for your labs, these people are the people you are going to hang out all year round (some labs takes about 12-14 hours of your time, 6-7 days, it is like your second home). So, their bonding is quite strong. Secluding yourself from that group will seem to be anti-social (or even being boycotted), but refraining yourselves of alcoholic drink would not seem to be anti-social, as you are just weak-to-alcohol (in Japanese), as I have read from several books, talking about the difference of culture of Japanese from us here.
As for alcohol, please make it a point that you may be excused from having alcohol, but remember to refuse them well. I think you will be learning that in the 6-month Japanese preparatory course. There are ways to refuse or reject offers. But just remember you should explain your conditions, I am sure they will understand you well.
Another point, about the entrance examination. I think you will be able to get the answer for the entrance examination. If possible, get several years' paper. The entrance examinations are easy in the sense that, there are usually a standard format, and standard scope for each of the subjects. If you are well prepared, then you can answer it well. But there are two main problems:
1. Time constraint - Japanese examination questions are well-known of their time-effectiveness. You can spare any time staring at the question without doing anything. The time is just enough for you to answer the questions.
2. Marking style - The marking style of the Japanese is very strict. It is strict to the sense that, if your final answer is incorrect, then you will not get any marks.
That's what I have heard from my friends and lecturers. Hope that it will be useful to share with you all.