It really varies by the school and thus, by the people who are running it. A pre-med society should, at the very least:
1) Create a social atmosphere for premeds to discuss issues pretaining to medical school and admissions
2) Provide community service and shadowing experiences
3) Bring in doctors to speak about the field, what to expect from real world medicine versus tv medicine, their views of the future, answer questions, etc
4) Educate freshmen/new premeds in regards to what they are going to undergo, what requirements they'll need to do, etc
5) Offer some sort of MCAT prep or general science tutoring
The problem is that most premed clubs just have people who are lazy as hell filling up positions and not doing anything to contribute to the goal of the club. They just want the title for their resume. They don't even want to show up at meetings half the time. So the content of the meetings goes downhill and with it, the club's reputation and chance at success.
Perhaps it is to be expected.
Anyway, you can attend meetings as a member, but don't expect your membership status to get you into medical school.