I think there are some neighborhood groups and dashcam groups in the larger US cities on social media sites like FB and Reddit but I can't say for sure as I don't fool with such stuff
I know there have been posts on the subject here long ago, but I don't know their outcome.
In a lot of places here Police ask that you voluntarily both register your cam with them and give them viewing access, ostensibly so they don't have to bother you to check on footage you might have. Many people do this and many don't. Part of the problem with that system is that with many cams, password access also gives the 'viewer' control of the cam and files, so they can do whatever they want with it including the deletion of video that might show errant actions by Police. There's also the question of ownership rights to your vids when you share them. News outlets and some YouTube channels will pay you for exclusive rights to your videos public exposure, and you can lose that option when you agree to allow Police access to your cam(s)
But this is getting a bit off course...
I have went back and viewed my dashcam recordings when local news has reported crimes or crashes at times and locations where I might have something useful, but otherwise I have no way of knowing if I can help. It would be good for there to be a website where we could hear about vids being needed and submit ours voluntarily but if it involves social media platforms count me out- I do not and will never do those even though most people do
Given the scale of things hosting such a website would be a costly venture but could be easily done by one of the tech giants like Google or Yahoo, but they're probably not interested.
My cams are mostly for me. I have shared parking recordings with Police when a neighbor's car disappeared and those helped sort the matter out
But in the same location there were two other incidents where I likely had useful footage if only someone would have told me something before it got overwritten so I could look for it. Another reason for having bigger cards; you may not need them but someone else might
Phil