Do you or anyone else looking to get involved in this group know any physicians personally (e.g. someone’s parent/aunt/uncle is a doctor)? Do your teachers know any physicians personally? Start with that person as a guest speaker and then ask them to refer you to other speakers. The speakers on that website you linked are almost certainly not going to travel to your school and talk for free (you’re going to have to pay for travel, accommodations, and speaker’s fee). Your classmate’s parent or their friend on the other hand will probably drive over to your school and talk at no cost to you.
What about local colleges/universities? Could you get the pre-med advisor there to come talk to you? Maybe even a college senior who is doing applications (a student may be better for the spring because then they’ll actually have been accepted to medical school which would be a better resource than just an applicant).
What is the closest medical school to you? Do they have any sort of medical student group that does outreach or education in any capacity? If they don’t, maybe they’d be interested in creating one and your school could be one of the first places they interact with. At my school there are groups that lead middle school students in doing cow heart or sheep eye dissections to learn some anatomy or they teach them how to suture on pig’s feet.
I think going to a conference that isn’t geared towards high school (or at most, college) students is going to be a poor use of your time. The presentations are going to be way over your heads. Or did you mean conferences specifically geared towards students?
Look around your local hospitals’ websites to see if there’s some sort of community outreach department or something. If not, worst case scenario just call the hospital’s most basic phone number and tell an operator what you’re trying to do and to connect you to whomever is best suited to handle that.
The County Medical Society has organizational access to the physicians in the county who may want to provide resources for your club. That might be speakers, suggestions, opportunities; maybe even funding or a scholarship.