what societies would you recommend joining? if you're interested in med specifically
Literally anything. I caution you to make your personality purely about med. You will be living and breathing medicine for the rest of your life. Take the time to now enjoy other aspects of university.
I too am going to enter medicine for postgrad. However, I decided against taking stereotypical premed majors (biochemistry, immunology) and explored other interests (biomed engineering, computer science, philosophy etc..). If you take a pathway that you yourself are interested it, you will be a more unique applicant and be able to bring a more interesting perspective to the table than someone who has studied medsci their whole life.
For postgrad entry, all that matters is GAMSAT, GPA and interviews. They don't care about your degree/major, so don't pick medsci just because you think it'll look good. Likewise, don't join med societies just because you think it will look good. Join them because you are genuinely interested.