Anyone arranging a Jazz jam will be aware of the wide range of skills available in a room full of fledgling Jazz musicians and will manage the event with everyone's needs in mind. If the playing standard is low, the tunes will be mostly diatonic, two, three of four chords and well known material like blues changes, Canteloupe Island, Watermelon Man, Summertime etc. As the standard improves, the tunes will get harder and harder but still with the ability of those present in mind. At the end of the day, no-one has any investment in managing a train crash so calling 'GIant Steps' for a room full of beginners would be stupid.
My advice? Learn the changes to a twelve bar blues in F and Bb and learn how to read a simple chord chart. A good starting point would be Jamey Aebersold's play-a-long series (I think a lot of them are on Spotify now, although the accompanying books won't be). And, if in doubt, talk to whoever is leading the session. They are generally approachable people and will answer all of your questions about 'protocols' and do all that they can to get you up there and playing.