For my sake, I picked up martial arts in college and would say that ~75% of my life long friends come from there. I have work-related friends, but there's a very clear difference between them and the people I've punched for years :D

I definitely agree with the shared struggle aspect and add the activity should also have "moments of down time" where you're still "doing" the activity, but you're sort of at a point of waiting. These are the moments where you get to find out someone's life, like if they've got a kid that plays a musical instrument or new policies at their own jobs.

One simple thing we did early on was simply ask some of the other people "what are you doing tonight?" after the class. At least at the college age, there's always a chance a handful of people will be down for going to a bar or pool hall afterwards.