just turn up to the friday one and ask the tutor if you can sit in. they might say no, but they'll probably say yes. after the first few weeks the number of kids attending declines dramatically.
Dont be so concerned if you go to one that isnt for biomed students. you'll be learning the exact same thing as everyone else, it just might be that 2nd or 3rd year biomed students are tutoring those classes whilst physio students, for example, might be tutoring others. your tutor will probably try to apply the information to a setting useful for your degree, but it wont make or break ur success in the course. The course is used as a basis for a huge array of degrees so the lecturer cant apply information to individual degrees. The pass sessions just serve to allow more 'like-minded' people to study together and get what their degree wants out of the course.
Pass groups are VERY lax.