Tony,
As an member of several meetup groups myself, including several centered around autism and/or neurodiversity, I found your post and title to be rather hurtful.
It seems like you may have misunderstood how groups work, and so I'll provide a bit of an explanation in case you or anyone else reading this thread may find it useful.
As Rodafina noted, meetup groups generally center around some sort of interest or activity.
In this case, for the group you mentioned, per their "about" page, they are a group centered around hiking, walking, and cycling in NY, NJ and CT.
It's a free public group (anyone can join, no costs to members or application needed), and a relatively large one, with several thousand members.
For today's event, it's described as a walk and talk. This is a pretty common type of meetup event - where folks meet at a given place to walk a predefined route. Participants can chat if they'd like, but there's no obligation to do so.
As is typical for large public groups, there were a large number of participants signed up (in the 70s), including both participants and their plus-one's.
This event happened to have a high ratio of first-timers, making up about 60% of the participants.
In such large groups, it's quite normal that the activity leaders would be focused on leading the event, and won't have the opportunity to get to talk to everyone.
With smaller groups (e.g. 10 or fewer) you'll be much more likely to see the group leaders, be they organizers or regulars, engage with new members.
But unless the objective is clearly stated as social in nature, and it's also stated that the organizers and/or regulars will actively engage with new members, I think it's unfair to expect that other participants would be actively initiating and be engaging in conversation with you.
As for your near-slip, and feeling ignored - why do you feel ignored? Should someone have said or done something? It seems like you're okay, and that's good. For someone to say something may have drawn attention to that and been potentially embarrassing, in which case not saying anything and moving on may be normal, especially if it was a one-off. If someone had multiple near-slips, then yes, I could see that being a situation where someone may express concern.
As for your statement about crossing an interstate, the walking route passes near an entrance for an interstate, but doesn't pass through one.
Route map provided:
141-99-141-75 Barclay Ave to St. Michael's Roman Catholic Church
Perhaps you saw signs indicating there was an interstate entrance nearby and thought you were on or crossing an interstate itself?
In any case, one should always take caution when crossing a road, even at a marked crosswalk and even if you have a green light. There's generally safety in numbers as well. From the way you described things, it sounds like you may have crossed on your own for some reason?
In any case, I'm sorry that you're upset, but it seems that this was in part due to some misunderstandings.
I noticed that you often mention feeling excluded in a variety of events and settings. I would suggest you see if your friend could join you for some events as an observer. They might be able to identify ways you may be able to better communicate with others, situations where you might have misunderstood things, and/or if there are things that you inadvertently say or do (including body language) which may be impacting how others approach (or don't approach) you.
