I try and keep the number of groups I'm in sensible enough so that I know each group and what it's for, if you reach the point you're over 50 groups it's unlikely you're truly active/engaging with more than three or four. I'm in a few groups with posting-means-sharing rules, although not necessarily to share images with these groups.
Here's an example, the blog The Phoblographer has an associated Flickr group..
And copying from the About page of the Flickr group..
Group Description
The Phoblographer is about the happenings in the world of photography/videography and how technology affects it and us everyday. If you have learned something from us, found our tips useful and maybe found your photography changed by something on this site, please share your photos with us.
Group Rules
You know and agree that all submissions can be used on ThePhoblographer.com in postings.
Submissions may be of anything and removal and moderation may be done at ThePhoblographer's will.
I don't find these unclear or unreasonable, and I came to that group expecting to find something similar. Other blog-related groups (e.g. the Strobist Flickr group) have similar rules that say photos posted in the Pool may be shared via the blog.
Following the Groups From Our Sponsors sidebar gets you to the commercial sponsor groups and their rules can be more interesting..
There don't seem to be as many sponsors as there used to be.